subsurface/core/divelist.c

// SPDX-License-Identifier: GPL-2.0
/* divelist.c */
/* core logic for the dive list -
 * accessed through the following interfaces:
 *
 * void process_loaded_dives();
 * void process_imported_dives(bool prefer_imported);
 * dive_trip_t *dive_trip_list;
 * unsigned int amount_selected;
 * void dump_selection(void)
 * void get_dive_gas(struct dive *dive, int *o2_p, int *he_p, int *o2low_p)
 * int total_weight(const struct dive *dive)
 * int get_divenr(const struct dive *dive)
 * int get_divesite_idx(const struct dive_site *ds)
 * int init_decompression(struct dive *dive)
 * void update_cylinder_related_info(struct dive *dive)
 * void dump_trip_list(void)
 * void insert_trip(dive_trip_t **dive_trip_p)
 * void insert_trip_dont_merge(dive_trip_t *dive_trip_p)
 * void unregister_trip(dive_trip_t *trip)
 * void free_trip(dive_trip_t *trip)
 * void remove_dive_from_trip(struct dive *dive)
 * void remove_dive_from_trip(struct dive *dive, bool was_autogen)
 * void add_dive_to_trip(struct dive *dive, dive_trip_t *trip)
 * dive_trip_t *create_and_hookup_trip_from_dive(struct dive *dive)
 * dive_trip_t *get_dives_to_autogroup(int start, int *from, int *to, bool *allocated)
 * void autogroup_dives(void)
 * void combine_trips(struct dive_trip *trip_a, struct dive_trip *trip_b)
 * dive_trip_t *combine_trips_create(struct dive_trip *trip_a, struct dive_trip *trip_b)
 * struct dive *unregister_dive(int idx)
 * void delete_single_dive(int idx)
 * void add_single_dive(int idx, struct dive *dive)
 * struct dive *merge_two_dives(struct dive *a, struct dive *b)
 * void select_dive(int idx)
 * void deselect_dive(int idx)
 * void mark_divelist_changed(int changed)
 * int unsaved_changes()
 * void remove_autogen_trips()
 * void sort_table(struct dive_table *table)
 * bool is_trip_before_after(const struct dive *dive, bool before)
 * void delete_dive_from_table(struct dive_table *table, int idx)
 */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <math.h>
#include "gettext.h"
#include <assert.h>
#include <zip.h>
#include <libxslt/transform.h>

#include "dive.h"
#include "subsurface-string.h"
#include "divelist.h"
#include "display.h"
#include "planner.h"
#include "qthelper.h"
#include "git-access.h"

static bool dive_list_changed = false;

bool autogroup = false;

dive_trip_t *dive_trip_list;

unsigned int amount_selected;

// We need to stop using globals, really.
struct dive_table downloadTable;

#if DEBUG_SELECTION_TRACKING
void dump_selection(void)
{
	int i;
	struct dive *dive;

	printf("currently selected are %u dives:", amount_selected);
	for_each_dive(i, dive) {
		if (dive->selected)
			printf(" %d", i);
	}
	printf("\n");
}
#endif

void set_autogroup(bool value)
{
	/* if we keep the UI paradigm, this needs to toggle
	 * the checkbox on the autogroup menu item */
	autogroup = value;
}

/*
 * Get "maximal" dive gas for a dive.
 * Rules:
 *  - Trimix trumps nitrox (highest He wins, O2 breaks ties)
 *  - Nitrox trumps air (even if hypoxic)
 * These are the same rules as the inter-dive sorting rules.
 */
void get_dive_gas(struct dive *dive, int *o2_p, int *he_p, int *o2max_p)
{
	int i;
	int maxo2 = -1, maxhe = -1, mino2 = 1000;


	for (i = 0; i < MAX_CYLINDERS; i++) {
		cylinder_t *cyl = dive->cylinder + i;
		int o2 = get_o2(cyl->gasmix);
		int he = get_he(cyl->gasmix);

		if (!is_cylinder_used(dive, i))
			continue;
		if (cylinder_none(cyl))
			continue;
		if (o2 > maxo2)
			maxo2 = o2;
		if (he > maxhe)
			goto newmax;
		if (he < maxhe)
			continue;
		if (o2 <= maxo2)
			continue;
	newmax:
		maxhe = he;
		mino2 = o2;
	}
	/* All air? Show/sort as "air"/zero */
	if ((!maxhe && maxo2 == O2_IN_AIR && mino2 == maxo2) ||
			(maxo2 == -1 && maxhe == -1 && mino2 == 1000))
		maxo2 = mino2 = 0;
	*o2_p = mino2;
	*he_p = maxhe;
	*o2max_p = maxo2;
}

int total_weight(const struct dive *dive)
{
	int i, total_grams = 0;

	if (dive)
		for (i = 0; i < MAX_WEIGHTSYSTEMS; i++)
			total_grams += dive->weightsystem[i].weight.grams;
	return total_grams;
}

static int active_o2(const struct dive *dive, const struct divecomputer *dc, duration_t time)
{
	struct gasmix gas = get_gasmix_at_time(dive, dc, time);
	return get_o2(gas);
}

/* calculate OTU for a dive - this only takes the first divecomputer into account */
static int calculate_otu(const struct dive *dive)
{
	int i;
	double otu = 0.0;
	const struct divecomputer *dc = &dive->dc;

	for (i = 1; i < dc->samples; i++) {
		int t;
		int po2;
		struct sample *sample = dc->sample + i;
		struct sample *psample = sample - 1;
		t = sample->time.seconds - psample->time.seconds;
		if (sample->setpoint.mbar) {
			po2 = sample->setpoint.mbar;
		} else {
			int o2 = active_o2(dive, dc, sample->time);
			po2 = lrint(o2 * depth_to_atm(sample->depth.mm, dive));
		}
		if (po2 >= 500)
			otu += pow((po2 - 500) / 1000.0, 0.83) * t / 30.0;
	}
	return lrint(otu);
}
/* calculate CNS for a dive - this only takes the first divecomputer into account */
int const cns_table[][3] = {
	/* po2, Maximum Single Exposure, Maximum 24 hour Exposure */
	{ 1600, 45 * 60, 150 * 60 },
	{ 1500, 120 * 60, 180 * 60 },
	{ 1400, 150 * 60, 180 * 60 },
	{ 1300, 180 * 60, 210 * 60 },
	{ 1200, 210 * 60, 240 * 60 },
	{ 1100, 240 * 60, 270 * 60 },
	{ 1000, 300 * 60, 300 * 60 },
	{ 900, 360 * 60, 360 * 60 },
	{ 800, 450 * 60, 450 * 60 },
	{ 700, 570 * 60, 570 * 60 },
	{ 600, 720 * 60, 720 * 60 }
};

/* Calculate the CNS for a single dive */
static double calculate_cns_dive(const struct dive *dive)
{
	int n;
	size_t j;
	const struct divecomputer *dc = &dive->dc;
	double cns = 0.0;

	/* Caclulate the CNS for each sample in this dive and sum them */
	for (n = 1; n < dc->samples; n++) {
		int t;
		int po2;
		struct sample *sample = dc->sample + n;
		struct sample *psample = sample - 1;
		t = sample->time.seconds - psample->time.seconds;
		if (sample->setpoint.mbar) {
			po2 = sample->setpoint.mbar;
		} else {
			int o2 = active_o2(dive, dc, sample->time);
			po2 = lrint(o2 * depth_to_atm(sample->depth.mm, dive));
		}
		/* CNS don't increse when below 500 matm */
		if (po2 < 500)
			continue;
		/* Find what table-row we should calculate % for */
		for (j = 1; j < sizeof(cns_table) / (sizeof(int) * 3); j++)
			if (po2 > cns_table[j][0])
				break;
		j--;
		cns += ((double)t) / ((double)cns_table[j][1]) * 100;
	}

	return cns;
}

/* this only gets called if dive->maxcns == 0 which means we know that
 * none of the divecomputers has tracked any CNS for us
 * so we calculated it "by hand" */
static int calculate_cns(struct dive *dive)
{
	int i, divenr;
	double cns = 0.0;
	timestamp_t last_starttime, last_endtime = 0;

	/* shortcut */
	if (dive->cns)
		return dive->cns;

	divenr = get_divenr(dive);
	i = divenr >= 0 ? divenr : dive_table.nr;
#if DECO_CALC_DEBUG & 2
	if (i >= 0 && i < dive_table.nr)
		printf("\n\n*** CNS for dive #%d %d\n", i, get_dive(i)->number);
	else
		printf("\n\n*** CNS for dive #%d\n", i);
#endif
	/* Look at next dive in dive list table and correct i when needed */
	while (i < dive_table.nr - 1) {
		struct dive *pdive = get_dive(i);
		if (!pdive || pdive->when > dive->when)
			break;
		i++;
	}
	/* Look at previous dive in dive list table and correct i when needed */
	while (i > 0) {
		struct dive *pdive = get_dive(i - 1);
		if (!pdive || pdive->when < dive->when)
			break;
		i--;
	}
#if DECO_CALC_DEBUG & 2
	printf("Dive number corrected to #%d\n", i);
#endif
	last_starttime = dive->when;
	/* Walk backwards to check previous dives - how far do we need to go back? */
	while (i--) {
		if (i == divenr && i > 0)
			i--;
#if DECO_CALC_DEBUG & 2
		printf("Check if dive #%d %d has to be considered as prev dive: ", i, get_dive(i)->number);
#endif
		struct dive *pdive = get_dive(i);
		/* we don't want to mix dives from different trips as we keep looking
		 * for how far back we need to go */
		if (dive->divetrip && pdive->divetrip != dive->divetrip) {
#if DECO_CALC_DEBUG & 2
			printf("No - other dive trip\n");
#endif
			continue;
		}
		if (!pdive || pdive->when >= dive->when || dive_endtime(pdive) + 12 * 60 * 60 < last_starttime) {
#if DECO_CALC_DEBUG & 2
			printf("No\n");
#endif
			break;
		}
		last_starttime = pdive->when;
#if DECO_CALC_DEBUG & 2
		printf("Yes\n");
#endif
	}
	/* Walk forward and add dives and surface intervals to CNS */
	while (++i < dive_table.nr) {
#if DECO_CALC_DEBUG & 2
		printf("Check if dive #%d %d will be really added to CNS calc: ", i, get_dive(i)->number);
#endif
		struct dive *pdive = get_dive(i);
		/* again skip dives from different trips */
		if (dive->divetrip && dive->divetrip != pdive->divetrip) {
#if DECO_CALC_DEBUG & 2
			printf("No - other dive trip\n");
#endif
			continue;
		}
		/* Don't add future dives */
		if (pdive->when >= dive->when) {
#if DECO_CALC_DEBUG & 2
			printf("No - future or same dive\n");
#endif
			break;
		}
		/* Don't add the copy of the dive itself */
		if (i == divenr) {
#if DECO_CALC_DEBUG & 2
			printf("No - copy of dive\n");
#endif
			continue;
		}
#if DECO_CALC_DEBUG & 2
		printf("Yes\n");
#endif

		/* CNS reduced with 90min halftime during surface interval */
		if (last_endtime)
			cns /= pow(2, (pdive->when - last_endtime) / (90.0 * 60.0));
#if DECO_CALC_DEBUG & 2
		printf("CNS after surface interval: %f\n", cns);
#endif

		cns += calculate_cns_dive(pdive);
#if DECO_CALC_DEBUG & 2
		printf("CNS after previous dive: %f\n", cns);
#endif

		last_starttime = pdive->when;
		last_endtime = dive_endtime(pdive);
	}

	/* CNS reduced with 90min halftime during surface interval */
	if (last_endtime)
		cns /= pow(2, (dive->when - last_endtime) / (90.0 * 60.0));
#if DECO_CALC_DEBUG & 2
	printf("CNS after last surface interval: %f\n", cns);
#endif

	cns += calculate_cns_dive(dive);
#if DECO_CALC_DEBUG & 2
	printf("CNS after dive: %f\n", cns);
#endif

	/* save calculated cns in dive struct */
	dive->cns = lrint(cns);
	return dive->cns;
}
/*
 * Return air usage (in liters).
 */
static double calculate_airuse(const struct dive *dive)
{
	int airuse = 0;
	int i;

	for (i = 0; i < MAX_CYLINDERS; i++) {
		pressure_t start, end;
		const cylinder_t *cyl = dive->cylinder + i;

		start = cyl->start.mbar ? cyl->start : cyl->sample_start;
		end = cyl->end.mbar ? cyl->end : cyl->sample_end;
		if (!end.mbar || start.mbar <= end.mbar) {
			// If a cylinder is used but we do not have info on amout of gas used
			// better not pretend we know the total gas use.
			// Eventually, logic should be fixed to compute average depth and total time
			// for those segments where cylinders with known pressure drop are breathed from.
			if (is_cylinder_used(dive, i))
				return 0.0;
			else
				continue;
		}

		airuse += gas_volume(cyl, start) - gas_volume(cyl, end);
	}
	return airuse / 1000.0;
}

/* this only uses the first divecomputer to calculate the SAC rate */
static int calculate_sac(const struct dive *dive)
{
	const struct divecomputer *dc = &dive->dc;
	double airuse, pressure, sac;
	int duration, meandepth;

	airuse = calculate_airuse(dive);
	if (!airuse)
		return 0;

	duration = dc->duration.seconds;
	if (!duration)
		return 0;

	meandepth = dc->meandepth.mm;
	if (!meandepth)
		return 0;

	/* Mean pressure in ATM (SAC calculations are in atm*l/min) */
	pressure = depth_to_atm(meandepth, dive);
	sac = airuse / pressure * 60 / duration;

	/* milliliters per minute.. */
	return lrint(sac * 1000);
}

/* for now we do this based on the first divecomputer */
static void add_dive_to_deco(struct deco_state *ds, struct dive *dive)
{
	struct divecomputer *dc = &dive->dc;
	struct gasmix gasmix = gasmix_air;
	int i;
	const struct event *ev = NULL, *evd = NULL;
	enum divemode_t current_divemode = UNDEF_COMP_TYPE;

	if (!dc)
		return;

	for (i = 1; i < dc->samples; i++) {
		struct sample *psample = dc->sample + i - 1;
		struct sample *sample = dc->sample + i;
		int t0 = psample->time.seconds;
		int t1 = sample->time.seconds;
		int j;

		for (j = t0; j < t1; j++) {
			int depth = interpolate(psample->depth.mm, sample->depth.mm, j - t0, t1 - t0);
			gasmix = get_gasmix(dive, dc, j, &ev, gasmix);
			add_segment(ds, depth_to_bar(depth, dive), gasmix, 1, sample->setpoint.mbar,
				get_current_divemode(&dive->dc, j, &evd, &current_divemode), dive->sac);
		}
	}
}

int get_divenr(const struct dive *dive)
{
	int i;
	const struct dive *d;
	// tempting as it may be, don't die when called with dive=NULL
	if (dive)
		for_each_dive(i, d) {
			if (d->id == dive->id) // don't compare pointers, we could be passing in a copy of the dive
				return i;
		}
	return -1;
}

int get_divesite_idx(const struct dive_site *ds)
{
	int i;
	const struct dive_site *d;
	// tempting as it may be, don't die when called with dive=NULL
	if (ds)
		for_each_dive_site(i, d) {
			if (d->uuid == ds->uuid) // don't compare pointers, we could be passing in a copy of the dive
				return i;
		}
	return -1;
}

static struct gasmix air = { .o2.permille = O2_IN_AIR, .he.permille = 0 };

/* take into account previous dives until there is a 48h gap between dives */
/* return last surface time before this dive or dummy value of 48h */
/* return negative surface time if dives are overlapping */
/* The place you call this function is likely the place where you want
 * to create the deco_state */
int init_decompression(struct deco_state *ds, struct dive *dive)
{
	int i, divenr = -1;
	int surface_time = 48 * 60 * 60;
	timestamp_t last_endtime = 0, last_starttime = 0;
	bool deco_init = false;
	double surface_pressure;

	if (!dive)
		return false;

	divenr = get_divenr(dive);
	i = divenr >= 0 ? divenr : dive_table.nr;
#if DECO_CALC_DEBUG & 2
	if (i >= 0 && i < dive_table.nr)
		printf("\n\n*** Init deco for dive #%d %d\n", i, get_dive(i)->number);
	else
		printf("\n\n*** Init deco for dive #%d\n", i);
#endif
	/* Look at next dive in dive list table and correct i when needed */
	while (i < dive_table.nr - 1) {
		struct dive *pdive = get_dive(i);
		if (!pdive || pdive->when > dive->when)
			break;
		i++;
	}
	/* Look at previous dive in dive list table and correct i when needed */
	while (i > 0) {
		struct dive *pdive = get_dive(i - 1);
		if (!pdive || pdive->when < dive->when)
			break;
		i--;
	}
#if DECO_CALC_DEBUG & 2
	printf("Dive number corrected to #%d\n", i);
#endif
	last_starttime = dive->when;
	/* Walk backwards to check previous dives - how far do we need to go back? */
	while (i--) {
		if (i == divenr && i > 0)
			i--;
#if DECO_CALC_DEBUG & 2
		printf("Check if dive #%d %d has to be considered as prev dive: ", i, get_dive(i)->number);
#endif
		struct dive *pdive = get_dive(i);
		/* we don't want to mix dives from different trips as we keep looking
		 * for how far back we need to go */
		if (dive->divetrip && pdive->divetrip != dive->divetrip) {
#if DECO_CALC_DEBUG & 2
			printf("No - other dive trip\n");
#endif
			continue;
		}
		if (!pdive || pdive->when >= dive->when || dive_endtime(pdive) + 48 * 60 * 60 < last_starttime) {
#if DECO_CALC_DEBUG & 2
			printf("No\n");
#endif
			break;
		}
		last_starttime = pdive->when;
#if DECO_CALC_DEBUG & 2
		printf("Yes\n");
#endif
	}
	/* Walk forward an add dives and surface intervals to deco */
	while (++i < dive_table.nr) {
#if DECO_CALC_DEBUG & 2
		printf("Check if dive #%d %d will be really added to deco calc: ", i, get_dive(i)->number);
#endif
		struct dive *pdive = get_dive(i);
		/* again skip dives from different trips */
		if (dive->divetrip && dive->divetrip != pdive->divetrip) {
#if DECO_CALC_DEBUG & 2
			printf("No - other dive trip\n");
#endif
			continue;
		}
		/* Don't add future dives */
		if (pdive->when >= dive->when) {
#if DECO_CALC_DEBUG & 2
			printf("No - future or same dive\n");
#endif
			break;
		}
		/* Don't add the copy of the dive itself */
		if (i == divenr) {
#if DECO_CALC_DEBUG & 2
			printf("No - copy of dive\n");
#endif
			continue;
		}
#if DECO_CALC_DEBUG & 2
		printf("Yes\n");
#endif

		surface_pressure = get_surface_pressure_in_mbar(pdive, true) / 1000.0;
		/* Is it the first dive we add? */
		if (!deco_init) {
#if DECO_CALC_DEBUG & 2
			printf("Init deco\n");
#endif
			clear_deco(ds, surface_pressure);
			deco_init = true;
#if DECO_CALC_DEBUG & 2
			printf("Tissues after init:\n");
			dump_tissues(ds);
#endif
		}
		else {
			surface_time = pdive->when - last_endtime;
			if (surface_time < 0) {
#if DECO_CALC_DEBUG & 2
				printf("Exit because surface intervall is %d\n", surface_time);
#endif
				return surface_time;
			}
			add_segment(ds, surface_pressure, air, surface_time, 0, dive->dc.divemode, prefs.decosac);
#if DECO_CALC_DEBUG & 2
			printf("Tissues after surface intervall of %d:%02u:\n", FRACTION(surface_time, 60));
			dump_tissues(ds);
#endif
		}

		add_dive_to_deco(ds, pdive);

		last_starttime = pdive->when;
		last_endtime = dive_endtime(pdive);
		clear_vpmb_state(ds);
#if DECO_CALC_DEBUG & 2
		printf("Tissues after added dive #%d:\n", pdive->number);
		dump_tissues(ds);
#endif
	}

	surface_pressure = get_surface_pressure_in_mbar(dive, true) / 1000.0;
	/* We don't have had a previous dive at all? */
	if (!deco_init) {
#if DECO_CALC_DEBUG & 2
		printf("Init deco\n");
#endif
		clear_deco(ds, surface_pressure);
#if DECO_CALC_DEBUG & 2
		printf("Tissues after no previous dive, surface time set to 48h:\n");
		dump_tissues(ds);
#endif
	}
	else {
		surface_time = dive->when - last_endtime;
		if (surface_time < 0) {
#if DECO_CALC_DEBUG & 2
			printf("Exit because surface intervall is %d\n", surface_time);
#endif
			return surface_time;
		}
		add_segment(ds, surface_pressure, air, surface_time, 0, dive->dc.divemode, prefs.decosac);
#if DECO_CALC_DEBUG & 2
		printf("Tissues after surface intervall of %d:%02u:\n", FRACTION(surface_time, 60));
		dump_tissues(ds);
#endif
	}

	// I do not dare to remove this call. We don't need the result but it might have side effects. Bummer.
	tissue_tolerance_calc(ds, dive, surface_pressure);
	return surface_time;
}

void update_cylinder_related_info(struct dive *dive)
{
	if (dive != NULL) {
		dive->sac = calculate_sac(dive);
		dive->otu = calculate_otu(dive);
		if (dive->maxcns == 0)
			dive->maxcns = calculate_cns(dive);
	}
}

#define MAX_GAS_STRING 80
#define UTF8_ELLIPSIS "\xE2\x80\xA6"

/* callers needs to free the string */
char *get_dive_gas_string(struct dive *dive)
{
	int o2, he, o2max;
	char *buffer = malloc(MAX_GAS_STRING);

	if (buffer) {
		get_dive_gas(dive, &o2, &he, &o2max);
		o2 = (o2 + 5) / 10;
		he = (he + 5) / 10;
		o2max = (o2max + 5) / 10;

		if (he)
			if (o2 == o2max)
				snprintf(buffer, MAX_GAS_STRING, "%d/%d", o2, he);
			else
				snprintf(buffer, MAX_GAS_STRING, "%d/%d" UTF8_ELLIPSIS "%d%%", o2, he, o2max);
		else if (o2)
			if (o2 == o2max)
				snprintf(buffer, MAX_GAS_STRING, "%d%%", o2);
			else
				snprintf(buffer, MAX_GAS_STRING, "%d" UTF8_ELLIPSIS "%d%%", o2, o2max);
		else
			strcpy(buffer, translate("gettextFromC", "air"));
	}
	return buffer;
}

/*
 * helper functions for dive_trip handling
 */
#ifdef DEBUG_TRIP
void dump_trip_list(void)
{
	dive_trip_t *trip;
	int i = 0;
	timestamp_t last_time = 0;

	for (trip = dive_trip_list; trip; trip = trip->next) {
		struct tm tm;
		utc_mkdate(trip->when, &tm);
		if (trip->when < last_time)
			printf("\n\ndive_trip_list OUT OF ORDER!!!\n\n\n");
		printf("%s trip %d to \"%s\" on %04u-%02u-%02u %02u:%02u:%02u (%d dives - %p)\n",
		       trip->autogen ? "autogen " : "",
		       ++i, trip->location,
		       tm.tm_year, tm.tm_mon + 1, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec,
		       trip->nrdives, trip);
		last_time = trip->when;
	}
	printf("-----\n");
}
#endif

/* insert the trip into the dive_trip_list - but ensure you don't have
 * two trips for the same date; but if you have, make sure you don't
 * keep the one with less information */
void insert_trip(dive_trip_t **dive_trip_p)
{
	dive_trip_t *dive_trip = *dive_trip_p;
	dive_trip_t **p = &dive_trip_list;
	dive_trip_t *trip;
	struct dive *divep;

	/* Walk the dive trip list looking for the right location.. */
	while ((trip = *p) != NULL && trip->when < dive_trip->when)
		p = &trip->next;

	if (trip && trip->when == dive_trip->when) {
		if (!trip->location)
			trip->location = dive_trip->location;
		if (!trip->notes)
			trip->notes = dive_trip->notes;
		divep = dive_trip->dives;
		while (divep) {
			add_dive_to_trip(divep, trip);
			divep = divep->next;
		}
		*dive_trip_p = trip;
	} else {
		dive_trip->next = trip;
		*p = dive_trip;
	}
#ifdef DEBUG_TRIP
	dump_trip_list();
#endif
}

/* same as insert_trip, but don't merge trips with the same date.
 * this is cruical for the merge undo-command, because there we
 * add a new trip with the same date and then remove the old one. */
void insert_trip_dont_merge(dive_trip_t *dive_trip)
{
	dive_trip_t **p = &dive_trip_list;
	dive_trip_t *trip;

	/* Walk the dive trip list looking for the right location.. */
	while ((trip = *p) != NULL && trip->when < dive_trip->when)
		p = &trip->next;

	dive_trip->next = trip;
	*p = dive_trip;
}

/* free resources associated with a trip structure */
void free_trip(dive_trip_t *trip)
{
	free(trip->location);
	free(trip->notes);
	free(trip);
}

/* remove trip from the trip-list, but don't free its memory.
 * caller takes ownership of the trip. */
void unregister_trip(dive_trip_t *trip)
{
	dive_trip_t **p, *tmp;

	assert(!trip->dives);

	/* Remove the trip from the list of trips */
	p = &dive_trip_list;
	while ((tmp = *p) != NULL) {
		if (tmp == trip) {
			*p = trip->next;
			break;
		}
		p = &tmp->next;
	}
}

static void delete_trip(dive_trip_t *trip)
{
	unregister_trip(trip);
	free_trip(trip);
}

void find_new_trip_start_time(dive_trip_t *trip)
{
	struct dive *dive = trip->dives;
	timestamp_t when = dive->when;

	while ((dive = dive->next) != NULL) {
		if (dive->when < when)
			when = dive->when;
	}
	trip->when = when;
}

/* check if we have a trip right before / after this dive */
bool is_trip_before_after(const struct dive *dive, bool before)
{
	int idx = get_idx_by_uniq_id(dive->id);
	if (before) {
		if (idx > 0 && get_dive(idx - 1)->divetrip)
			return true;
	} else {
		if (idx < dive_table.nr - 1 && get_dive(idx + 1)->divetrip)
			return true;
	}
	return false;
}

struct dive *first_selected_dive()
{
	int idx;
	struct dive *d;

	for_each_dive (idx, d) {
		if (d->selected)
			return d;
	}
	return NULL;
}

struct dive *last_selected_dive()
{
	int idx;
	struct dive *d, *ret = NULL;

	for_each_dive (idx, d) {
		if (d->selected)
			ret = d;
	}
	return ret;
}

/* remove a dive from the trip it's associated to, but don't delete the
 * trip if this was the last dive in the trip. the caller is responsible
 * for removing the trip, if the trip->nrdives went to 0.
 */
struct dive_trip *unregister_dive_from_trip(struct dive *dive, short was_autogen)
{
	struct dive *next, **pprev;
	dive_trip_t *trip = dive->divetrip;

	if (!trip)
		return NULL;

	/* Remove the dive from the trip's list of dives */
	next = dive->next;
	pprev = dive->pprev;
	*pprev = next;
	if (next)
		next->pprev = pprev;

	dive->divetrip = NULL;
	if (was_autogen)
		dive->tripflag = TF_NONE;
	else
		dive->tripflag = NO_TRIP;
	assert(trip->nrdives > 0);
	--trip->nrdives;
	if (trip->nrdives > 0 && trip->when == dive->when)
		find_new_trip_start_time(trip);
	return trip;
}

void remove_dive_from_trip(struct dive *dive, short was_autogen)
{
	struct dive_trip *trip = unregister_dive_from_trip(dive, was_autogen);
	if (trip && trip->nrdives == 0)
		delete_trip(trip);
}

void add_dive_to_trip(struct dive *dive, dive_trip_t *trip)
{
	if (dive->divetrip == trip)
		return;
	remove_dive_from_trip(dive, false);
	trip->nrdives++;
	dive->divetrip = trip;
	dive->tripflag = ASSIGNED_TRIP;

	/* Add it to the trip's list of dives*/
	dive->next = trip->dives;
	if (dive->next)
		dive->next->pprev = &dive->next;
	trip->dives = dive;
	dive->pprev = &trip->dives;

	if (dive->when && trip->when > dive->when)
		trip->when = dive->when;
}

dive_trip_t *alloc_trip(void)
{
	return calloc(1, sizeof(dive_trip_t));
}

dive_trip_t *create_trip_from_dive(struct dive *dive)
{
	dive_trip_t *trip;

	trip = alloc_trip();
	trip->when = dive->when;
	trip->location = copy_string(get_dive_location(dive));

	return trip;
}

dive_trip_t *create_and_hookup_trip_from_dive(struct dive *dive)
{
	dive_trip_t *dive_trip = alloc_trip();

	dive_trip = create_trip_from_dive(dive);
	insert_trip(&dive_trip);

	dive->tripflag = IN_TRIP;
	add_dive_to_trip(dive, dive_trip);
	return dive_trip;
}

/*
 * Collect dives for auto-grouping. Pass in first dive which should be checked.
 * Returns range of dives that should be autogrouped and trip it should be
 * associated to. If the returned trip was newly allocated, the last bool
 * is set to true. Caller still has to register it in the system. Note
 * whereas this looks complicated - it is needed by the undo-system, which
 * manually injects the new trips. If there are no dives to be autogrouped,
 * return NULL.
 */
dive_trip_t *get_dives_to_autogroup(int start, int *from, int *to, bool *allocated)
{
	int i;
	struct dive *dive, *lastdive = NULL;

	/* Find first dive that should be merged and remember any previous
	 * dive that could be merged into.
	 */
	for (i = start; (dive = get_dive(i)) != NULL; i++) {
		dive_trip_t *trip;

		if (dive->divetrip) {
			lastdive = dive;
			continue;
		}

		if (!DIVE_NEEDS_TRIP(dive)) {
			lastdive = NULL;
			continue;
		}

		/* We found a dive, let's see if we have to allocate a new trip */
		if (!lastdive || dive->when >= lastdive->when + TRIP_THRESHOLD) {
			/* allocate new trip */
			trip = create_trip_from_dive(dive);
			trip->autogen = true;
			*allocated = true;
		} else {
			/* use trip of previous dive */
			trip = lastdive->divetrip;
			*allocated = false;
		}

		// Now, find all dives that will be added to this trip
		lastdive = dive;
		*from = i;
		for (*to = *from + 1; (dive = get_dive(*to)) != NULL; (*to)++) {
			if (dive->divetrip || !DIVE_NEEDS_TRIP(dive) ||
			    dive->when >= lastdive->when + TRIP_THRESHOLD)
				break;
			if (get_dive_location(dive) && !trip->location)
				trip->location = copy_string(get_dive_location(dive));
			lastdive = dive;
		}
		return trip;
	}

	/* Did not find anyhting - mark as end */
	return NULL;
}

/*
 * Walk the dives from the oldest dive, and see if we can autogroup them
 */
void autogroup_dives(void)
{
	int from, to;
	dive_trip_t *trip;
	int i, j;
	bool alloc;

	for(i = 0; (trip = get_dives_to_autogroup(i, &from, &to, &alloc)) != NULL; i = to) {
		/* If this was newly allocated, add trip to list */
		if (alloc)
			insert_trip(&trip);
		for (j = from; j < to; ++j)
			add_dive_to_trip(get_dive(j), trip);
	}
#ifdef DEBUG_TRIP
	dump_trip_list();
#endif
}

static void unregister_dive_from_table(struct dive_table *table, int idx)
{
	int i;
	for (i = idx; i < table->nr - 1; i++)
		table->dives[i] = table->dives[i + 1];
	table->dives[--table->nr] = NULL;
}

/* Remove a dive from a dive table. This assumes that the
 * dive was already removed from any trip and deselected.
 * It simply shrinks the table and frees the trip */
void delete_dive_from_table(struct dive_table *table, int idx)
{
	free_dive(table->dives[idx]);
	unregister_dive_from_table(table, idx);
}

/* this removes a dive from the dive table and trip-list but doesn't
 * free the resources associated with the dive. It returns a pointer
 * to the unregistered dive. */
struct dive *unregister_dive(int idx)
{
	struct dive *dive = get_dive(idx);
	if (!dive)
		return NULL; /* this should never happen */
	remove_dive_from_trip(dive, false);
	if (dive->selected)
		deselect_dive(idx);
	unregister_dive_from_table(&dive_table, idx);
	return dive;
}

/* this implements the mechanics of removing the dive from the table,
 * but doesn't deal with updating dive trips, etc */
void delete_single_dive(int idx)
{
	struct dive *dive = unregister_dive(idx);
	free_dive(dive);
}

struct dive **grow_dive_table(struct dive_table *table)
{
	int nr = table->nr, allocated = table->allocated;
	struct dive **dives = table->dives;

	if (nr >= allocated) {
		allocated = (nr + 32) * 3 / 2;
		dives = realloc(dives, allocated * sizeof(struct dive *));
		if (!dives)
			exit(1);
		table->dives = dives;
		table->allocated = allocated;
	}
	return dives;
}

/* get the index where we want to insert the dive so that everything stays
 * ordered reverse-chronologically */
int dive_get_insertion_index(struct dive *dive)
{
	for (int i = 0; i < dive_table.nr; i++) {
		if (dive->when <= dive_table.dives[i]->when)
			return i;
	}
	return dive_table.nr;
}

/* add a dive at the given index. if the index is negative, the dive will
 * be added according to reverse chronological order */
void add_single_dive(int idx, struct dive *dive)
{
	int i;
	if (idx < 0)
		idx = dive_get_insertion_index(dive);
	grow_dive_table(&dive_table);
	dive_table.nr++;
	if (dive->selected)
		amount_selected++;

	for (i = idx; i < dive_table.nr; i++) {
		struct dive *tmp = dive_table.dives[i];
		dive_table.dives[i] = dive;
		dive = tmp;
	}
}

bool consecutive_selected()
{
	struct dive *d;
	int i;
	bool consecutive = true;
	bool firstfound = false;
	bool lastfound = false;

	if (amount_selected == 0 || amount_selected == 1)
		return true;

	for_each_dive(i, d) {
		if (d->selected) {
			if (!firstfound)
				firstfound = true;
			else if (lastfound)
				consecutive = false;
		} else if (firstfound) {
			lastfound = true;
		}
	}
	return consecutive;
}

/*
 * Merge two dives. 'a' is always before 'b' in the dive list
 * (and thus in time).
 */
struct dive *merge_two_dives(struct dive *a, struct dive *b)
{
	struct dive *res;
	int i, j, nr, nrdiff;
	int id;

	if (!a || !b)
		return NULL;

	id = a->id;
	i = get_divenr(a);
	j = get_divenr(b);
	if (i < 0 || j < 0)
		// something is wrong with those dives. Bail
		return NULL;
	res = merge_dives(a, b, b->when - a->when, false, NULL);
	if (!res)
		return NULL;

	/*
	 * If 'a' and 'b' were numbered, and in proper order,
	 * then the resulting dive will get the first number,
	 * and the subsequent dives will be renumbered by the
	 * difference.
	 *
	 * So if you had a dive list  1 3 6 7 8, and you
	 * merge 1 and 3, the resulting numbered list will
	 * be 1 4 5 6, because we assume that there were
	 * some missing dives (originally dives 4 and 5),
	 * that now will still be missing (dives 2 and 3
	 * in the renumbered world).
	 *
	 * Obviously the normal case is that everything is
	 * consecutive, and the difference will be 1, so the
	 * above example is not supposed to be normal.
	 */
	nrdiff = 0;
	nr = a->number;
	if (a->number && b->number > a->number) {
		res->number = nr;
		nrdiff = b->number - nr;
	}

	add_single_dive(i, res);
	delete_single_dive(i + 1);
	delete_single_dive(j);
	// now make sure that we keep the id of the first dive.
	// why?
	// because this way one of the previously selected ids is still around
	res->id = id;

	// renumber dives from merged one in advance by difference between
	// merged dives numbers. Do not renumber if actual number is zero.
	for (; j < dive_table.nr; j++) {
		struct dive *dive = dive_table.dives[j];
		int newnr;

		if (!dive->number)
			continue;
		newnr = dive->number - nrdiff;

		/*
		 * Don't renumber stuff that isn't in order!
		 *
		 * So if the new dive number isn't larger than the
		 * previous dive number, just stop here.
		 */
		if (newnr <= nr)
			break;
		dive->number = newnr;
		nr = newnr;
	}

	mark_divelist_changed(true);
	return res;
}

void select_dive(int idx)
{
	struct dive *dive = get_dive(idx);
	if (dive) {
		/* never select an invalid dive that isn't displayed */
		if (!dive->selected) {
			dive->selected = 1;
			amount_selected++;
		}
		selected_dive = idx;
	}
}

void deselect_dive(int idx)
{
	struct dive *dive = get_dive(idx);
	if (dive && dive->selected) {
		dive->selected = 0;
		if (amount_selected)
			amount_selected--;
		if (selected_dive == idx && amount_selected > 0) {
			/* pick a different dive as selected */
			while (--selected_dive >= 0) {
				dive = get_dive(selected_dive);
				if (dive && dive->selected)
					return;
			}
			selected_dive = idx;
			while (++selected_dive < dive_table.nr) {
				dive = get_dive(selected_dive);
				if (dive && dive->selected)
					return;
			}
		}
		if (amount_selected == 0)
			selected_dive = -1;
	}
}

void deselect_dives_in_trip(struct dive_trip *trip)
{
	struct dive *dive;
	if (!trip)
		return;
	for (dive = trip->dives; dive; dive = dive->next)
		deselect_dive(get_divenr(dive));
}

void select_dives_in_trip(struct dive_trip *trip)
{
	struct dive *dive;
	if (!trip)
		return;
	for (dive = trip->dives; dive; dive = dive->next)
		if (!dive->hidden_by_filter)
			select_dive(get_divenr(dive));
}

void filter_dive(struct dive *d, bool shown)
{
	if (!d)
		return;
	d->hidden_by_filter = !shown;
	if (!shown && d->selected)
		deselect_dive(get_divenr(d));
}


/* This only gets called with non-NULL trips.
 * It does not combine notes or location, just picks the first one
 * (or the second one if the first one is empty */
void combine_trips(struct dive_trip *trip_a, struct dive_trip *trip_b)
{
	if (empty_string(trip_a->location) && trip_b->location) {
		free(trip_a->location);
		trip_a->location = strdup(trip_b->location);
	}
	if (empty_string(trip_a->notes) && trip_b->notes) {
		free(trip_a->notes);
		trip_a->notes = strdup(trip_b->notes);
	}
	/* this also removes the dives from trip_b and eventually
	 * calls delete_trip(trip_b) when the last dive has been moved */
	while (trip_b->dives)
		add_dive_to_trip(trip_b->dives, trip_a);
}

/* Out of two strings, copy the string that is not empty (if any). */
static char *copy_non_empty_string(const char *a, const char *b)
{
	return copy_string(empty_string(b) ? a : b);
}

/* Combine trips new. This combines two trips, generating a
 * new trip. To support undo, we have to preserve the old trips. */
dive_trip_t *combine_trips_create(struct dive_trip *trip_a, struct dive_trip *trip_b)
{
	dive_trip_t *trip;

	trip = alloc_trip();
	trip->when = trip_a->when;
	trip->location = copy_non_empty_string(trip_a->location, trip_b->location);
	trip->notes = copy_non_empty_string(trip_a->notes, trip_b->notes);

	return trip;
}

void mark_divelist_changed(bool changed)
{
	if (dive_list_changed == changed)
		return;
	dive_list_changed = changed;
	updateWindowTitle();
}

int unsaved_changes()
{
	return dive_list_changed;
}

void remove_autogen_trips()
{
	int i;
	struct dive *dive;

	for_each_dive(i, dive) {
		dive_trip_t *trip = dive->divetrip;

		if (trip && trip->autogen)
			remove_dive_from_trip(dive, true);
	}
}

/*
 * When adding dives to the dive table, we try to renumber
 * the new dives based on any old dives in the dive table.
 *
 * But we only do it if:
 *
 *  - there are no dives in the dive table
 *
 *  OR
 *
 *  - the last dive in the old dive table was numbered
 *
 *  - all the new dives are strictly at the end (so the
 *    "last dive" is at the same location in the dive table
 *    after re-sorting the dives.
 *
 *  - none of the new dives have any numbers
 *
 * This catches the common case of importing new dives from
 * a dive computer, and gives them proper numbers based on
 * your old dive list. But it tries to be very conservative
 * and not give numbers if there is *any* question about
 * what the numbers should be - in which case you need to do
 * a manual re-numbering.
 */
static void try_to_renumber(int preexisting)
{
	int i, nr;
	struct dive *last = get_dive(preexisting - 1);

	/*
	 * If there was a last dive, but it didn't have
	 * a number, give up.
	 */
	if (last && !last->number)
		return;

	/*
	 * If any of the new dives already had a number,
	 * we'll have to do a manual renumbering.
	 */
	for (i = preexisting; i < dive_table.nr; i++) {
		struct dive *dive = get_dive(i);
		if (dive->number)
			return;
	}

	/*
	 * Ok, renumber..
	 */
	if (last)
		nr = last->number;
	else
		nr = 0;
	for (i = preexisting; i < dive_table.nr; i++) {
		struct dive *dive = get_dive(i);
		dive->number = ++nr;
	}
}

void process_loaded_dives()
{
	int i;
	struct dive *dive;

	/* Register dive computer nick names */
	for_each_dive(i, dive)
		set_dc_nickname(dive);

	sort_table(&dive_table);
}

/*
 * Merge subsequent dives in a table, if mergeable. This assumes
 * that the dives are neither selected, not part of a trip, as
 * is the case of freshly imported dives.
 */
static void merge_imported_dives(struct dive_table *table)
{
	int i;
	for (i = 1; i < table->nr; i++) {
		struct dive *prev = table->dives[i - 1];
		struct dive *dive = table->dives[i];
		struct dive *merged;

		/* only try to merge overlapping dives - or if one of the dives has
		 * zero duration (that might be a gps marker from the webservice) */
		if (prev->duration.seconds && dive->duration.seconds &&
		    dive_endtime(prev) < dive->when)
			continue;

		merged = try_to_merge(prev, dive, false);
		if (!merged)
			continue;

		/* Overwrite the first of the two dives and remove the second */
		free_dive(prev);
		table->dives[i - 1] = merged;
		delete_dive_from_table(table, i);

		/* Redo the new 'i'th dive */
		i--;
	}
}

/*
 * Try to merge a new dive into the dive at position idx. Return
 * true on success. On success, the dive to add and the old dive
 * will be deleted. On failure, they are untouched.
 * If "prefer_imported" is true, use data of the new dive.
 */
static bool try_to_merge_into(struct dive *dive_to_add, int idx, bool prefer_imported)
{
	struct dive *old_dive = dive_table.dives[idx];
	struct dive_trip *trip = old_dive->divetrip;
	struct dive *merged = try_to_merge(old_dive, dive_to_add, prefer_imported);
	if (!merged)
		return false;

	merged->id = old_dive->id;
	merged->selected = old_dive->selected;
	dive_table.dives[idx] = merged;
	if (trip) {
		remove_dive_from_trip(old_dive, false);
		add_dive_to_trip(merged, trip);
	}
	free_dive(old_dive);
	free_dive(dive_to_add);

	return true;
}

/*
 * Add imported dive to global dive table. Overlapping dives will
 * be merged if possible. If prefer_imported is true, data of the
 * new dives are prioritized in such a case.
 * If downloaded is true, only the divecomputer of the first dive
 * will be considered, as it is assumed that all dives come from
 * the same computer.
 * Note: the dives in import_table are consumed! On return import_table
 * has size 0.
 */
void process_imported_dives(struct dive_table *import_table, bool prefer_imported, bool downloaded)
{
	int i, j;
	int preexisting;
	bool sequence_changed = false;

	/* If no dives were imported, don't bother doing anything */
	if (!import_table->nr)
		return;

	/* check if we need a nickname for the divecomputer for newly downloaded dives;
	 * since we know they all came from the same divecomputer we just check for the
	 * first one */
	if (downloaded)
		set_dc_nickname(import_table->dives[0]);
	else
		/* they aren't downloaded, so record / check all new ones */
		for (i = 0; i < import_table->nr; i++)
			set_dc_nickname(import_table->dives[i]);

	/* Sort the table of dives to be imported and combine mergable dives */
	sort_table(import_table);
	merge_imported_dives(import_table);

	/* Merge newly imported dives into the dive table.
	 * Since both lists (old and new) are sorted, we can step
	 * through them concurrently and locate the insertions points.
	 * Once found, check if the new dive can be merged in the
	 * previous or next dive.
	 * Note that this doesn't consider pathological cases such as:
	 *  - New dive "connects" two old dives (turn three into one).
	 *  - New dive can not be merged into adjacent but some further dive.
	 */
	j = 0; /* Index in old dives */
	preexisting = dive_table.nr; /* Remember old size for renumbering */
	for (i = 0; i < import_table->nr; i++) {
		struct dive *dive_to_add = import_table->dives[i];

		/* Find insertion point. */
		while (j < dive_table.nr && dive_table.dives[j]->when < dive_to_add->when)
			j++;

		/* Try to merge into previous dive. */
		if (j > 0 && dive_endtime(dive_table.dives[j - 1]) > dive_to_add->when) {
			if (try_to_merge_into(dive_to_add, j - 1, prefer_imported))
				continue;
		}

		/* That didn't merge into the previous dive. If we're
		 * at the end of the dive table, quit the loop and add
		 * all new dives at the end. */
		if (j >= dive_table.nr)
			break;

		/* Try to merge into next dive. */
		if (dive_endtime(dive_to_add) > dive_table.dives[j]->when) {
			if (try_to_merge_into(dive_to_add, j, prefer_imported))
				continue;
		}

		/* We couldnt merge dives, add at the given position. */
		add_single_dive(j, dive_to_add);
		j++;
		sequence_changed = true;
	}

	/* If there are still dives to add, add them at the end of the dive table. */
	for ( ; i <  import_table->nr; i++)
		add_single_dive(dive_table.nr, import_table->dives[i]);

	/* we took care of all dives, clean up the import table */
	import_table->nr = 0;

	/* If the sequence wasn't changed, renumber */
	if (!sequence_changed)
		try_to_renumber(preexisting);

	mark_divelist_changed(true);
}

/* return the number a dive gets when inserted at the given index.
 * this function is supposed to be called *before* a dive was added.
 * this returns:
 * 	- 1 for an empty log
 * 	- last_nr+1 for addition at end of log (if last dive had a number)
 * 	- 0 for all other cases
 */
int get_dive_nr_at_idx(int idx)
{
	if (dive_table.nr == 0)
		return 1;
	if (idx >= dive_table.nr) {
		struct dive *last_dive = get_dive(dive_table.nr - 1);
		return last_dive->number ? last_dive->number + 1 : 0;
	}
	return 0;
}

void set_dive_nr_for_current_dive()
{
	if (dive_table.nr == 1)
		current_dive->number = 1;
	else if (selected_dive == dive_table.nr - 1 && get_dive(dive_table.nr - 2)->number)
		current_dive->number = get_dive(dive_table.nr - 2)->number + 1;
}

static int min_datafile_version;

int get_min_datafile_version()
{
	return min_datafile_version;
}

void reset_min_datafile_version()
{
	min_datafile_version = 0;
}

void report_datafile_version(int version)
{
	if (min_datafile_version == 0 || min_datafile_version > version)
		min_datafile_version = version;
}

int get_dive_id_closest_to(timestamp_t when)
{
	int i;
	int nr = dive_table.nr;

	// deal with pathological cases
	if (nr == 0)
		return 0;
	else if (nr == 1)
		return dive_table.dives[0]->id;

	for (i = 0; i < nr && dive_table.dives[i]->when <= when; i++)
		; // nothing

	// again, capture the two edge cases first
	if (i == nr)
		return dive_table.dives[i - 1]->id;
	else if (i == 0)
		return dive_table.dives[0]->id;

	if (when - dive_table.dives[i - 1]->when < dive_table.dives[i]->when - when)
		return dive_table.dives[i - 1]->id;
	else
		return dive_table.dives[i]->id;
}

void clear_dive_file_data()
{
	while (dive_table.nr)
		delete_single_dive(0);
	while (dive_site_table.nr)
		delete_dive_site(get_dive_site(0)->uuid);

	clear_dive(&displayed_dive);
	clear_dive_site(&displayed_dive_site);

	reset_min_datafile_version();
	saved_git_id = "";
}

static int sortfn(const void *_a, const void *_b)
{
	const struct dive *a = (const struct dive *)*(void **)_a;
	const struct dive *b = (const struct dive *)*(void **)_b;

	if (a->when < b->when)
		return -1;
	if (a->when > b->when)
		return 1;
	return 0;
}

void sort_table(struct dive_table *table)
{
	qsort(table->dives, table->nr, sizeof(struct dive *), sortfn);
}

/*
 * Calculate surface interval for dive starting at "when". Currently, we
 * might display dives which are not yet in the divelist, therefore the
 * input parameter is a timestamp.
 * If the given dive starts during a different dive, the surface interval
 * is 0. If we can't determine a surface interval (first dive), <0 is
 * returned. This does *not* consider pathological cases such as dives
 * that happened inside other dives. The interval will always be calculated
 * with respect to the dive that started previously.
 */
timestamp_t get_surface_interval(timestamp_t when)
{
	int i;
	timestamp_t prev_end;

	/* find previous dive. might want to use a binary search. */
	for (i = dive_table.nr - 1; i >= 0; --i) {
		if (dive_table.dives[i]->when < when)
			break;
	}
	if (i < 0)
		return -1;

	prev_end = dive_endtime(dive_table.dives[i]);
	if (prev_end > when)
		return 0;
	return when - prev_end;
}