/*
 * Copyright (C) 2005 Junio C Hamano
 */
#include <sys/types.h>
#include <sys/wait.h>
#include "cache.h"
#include "diff.h"

static char *diff_cmd = "diff -L'k/%s' -L'l/%s'";
static char *diff_opts = "-pu";

static const char *external_diff(void)
{
        static char *external_diff_cmd = NULL;
        static int done_preparing = 0;

        if (done_preparing)
                return external_diff_cmd;

        /*
         * Default values above are meant to match the
         * Linux kernel development style.  Examples of
         * alternative styles you can specify via environment
         * variables are:
         *
         * GIT_DIFF_CMD="diff -L '%s' -L '%s'"
         * GIT_DIFF_OPTS="-c";
         */
        if (getenv("GIT_EXTERNAL_DIFF"))
                external_diff_cmd = getenv("GIT_EXTERNAL_DIFF");

        /* In case external diff fails... */
        diff_cmd = getenv("GIT_DIFF_CMD") ? : diff_cmd;
        diff_opts = getenv("GIT_DIFF_OPTS") ? : diff_opts;

        done_preparing = 1;
        return external_diff_cmd;
}

/* Help to copy the thing properly quoted for the shell safety.
 * any single quote is replaced with '\'', and the caller is
 * expected to enclose the result within a single quote pair.
 *
 * E.g.
 *  original     sq_expand     result
 *  name     ==> name      ==> 'name'
 *  a b      ==> a b       ==> 'a b'
 *  a'b      ==> a'\''b    ==> 'a'\''b'
 */
static char *sq_expand(const char *src)
{
        static char *buf = NULL;
        int cnt, c;
        const char *cp;
        char *bp;

        /* count bytes needed to store the quoted string. */ 
        for (cnt = 1, cp = src; *cp; cnt++, cp++)
                if (*cp == '\'')
                        cnt += 3;

        if (! (buf = malloc(cnt)))
            return buf;
        bp = buf;
        while ((c = *src++)) {
                if (c != '\'')
                        *bp++ = c;
                else {
                        bp = strcpy(bp, "'\\''");
                        bp += 4;
                }
        }
        *bp = 0;
        return buf;
}

static struct diff_tempfile {
        const char *name;
        char hex[41];
        char mode[10];
        char tmp_path[50];
} diff_temp[2];

static void builtin_diff(const char *name,
                         struct diff_tempfile *temp)
{
        static char *diff_arg  = "'%s' '%s'";
        const char *name_1_sq = sq_expand(temp[0].name);
        const char *name_2_sq = sq_expand(temp[1].name);
        const char *name_sq = sq_expand(name);

        /* diff_cmd and diff_arg have 4 %s in total which makes
         * the sum of these strings 8 bytes larger than required.
         * we use 2 spaces around diff-opts, and we need to count
         * terminating NUL, so we subtract 5 here.
         */
        int cmd_size = (strlen(diff_cmd) + 
                        strlen(name_sq) * 2 +
                        strlen(diff_opts) +
                        strlen(diff_arg) +
                        strlen(name_1_sq) + strlen(name_2_sq)
                        - 5);
        char *cmd = malloc(cmd_size);
        int next_at = 0;

        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            diff_cmd, name_sq, name_sq);
        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            " %s ", diff_opts);
        next_at += snprintf(cmd+next_at, cmd_size-next_at,
                            diff_arg, name_1_sq, name_2_sq);
        execlp("/bin/sh","sh", "-c", cmd, NULL);
}

static void prepare_temp_file(const char *name,
                              struct diff_tempfile *temp,
                              struct diff_spec *one)
{
        static unsigned char null_sha1[20] = { 0, };

        if (!one->file_valid) {
        not_a_valid_file:
                temp->name = "/dev/null";
                strcpy(temp->hex, ".");
                strcpy(temp->mode, ".");
                return;
        }

        if (one->sha1_valid &&
            !memcmp(one->u.sha1, null_sha1, sizeof(null_sha1))) {
                one->sha1_valid = 0;
                one->u.name = name;
        }

        if (!one->sha1_valid) {
                struct stat st;
                temp->name = one->u.name;
                if (stat(temp->name, &st) < 0) {
                        if (errno == ENOENT)
                                goto not_a_valid_file;
                        die("stat(%s): %s", temp->name, strerror(errno));
                }
                strcpy(temp->hex, ".");
                sprintf(temp->mode, "%06o",
                        S_IFREG |ce_permissions(st.st_mode));
        }
        else {
                int fd;
                void *blob;
                char type[20];
                unsigned long size;

                blob = read_sha1_file(one->u.sha1, type, &size);
                if (!blob || strcmp(type, "blob"))
                        die("unable to read blob object for %s (%s)",
                            name, sha1_to_hex(one->u.sha1));

                strcpy(temp->tmp_path, ".diff_XXXXXX");
                fd = mkstemp(temp->tmp_path);
                if (fd < 0)
                        die("unable to create temp-file");
                if (write(fd, blob, size) != size)
                        die("unable to write temp-file");
                close(fd);
                free(blob);
                temp->name = temp->tmp_path;
                strcpy(temp->hex, sha1_to_hex(one->u.sha1));
                temp->hex[40] = 0;
                sprintf(temp->mode, "%06o", one->mode);
        }
}

static void remove_tempfile(void)
{
        int i;

        for (i = 0; i < 2; i++)
                if (diff_temp[i].name == diff_temp[i].tmp_path) {
                        unlink(diff_temp[i].name);
                        diff_temp[i].name = NULL;
                }
}

/* An external diff command takes:
 *
 * diff-cmd name infile1 infile1-sha1 infile1-mode \
 *               infile2 infile2-sha1 infile2-mode.
 *
 */
void run_external_diff(const char *name,
                       struct diff_spec *one,
                       struct diff_spec *two)
{
        struct diff_tempfile *temp = diff_temp;
        int pid, status;
        static int atexit_asked = 0;

        prepare_temp_file(name, &temp[0], one);
        prepare_temp_file(name, &temp[1], two);
        if (! atexit_asked &&
            (temp[0].name == temp[0].tmp_path ||
             temp[1].name == temp[1].tmp_path)) {
                atexit_asked = 1;
                atexit(remove_tempfile);
        }

        fflush(NULL);
        pid = fork();
        if (pid < 0)
                die("unable to fork");
        if (!pid) {
                const char *pgm = external_diff();
                if (pgm)
                        execlp(pgm, pgm,
                               name,
                               temp[0].name, temp[0].hex, temp[0].mode,
                               temp[1].name, temp[1].hex, temp[1].mode,
                               NULL);
                /*
                 * otherwise we use the built-in one.
                 */
                builtin_diff(name, temp);
                exit(0);
        }
        if (waitpid(pid, &status, 0) < 0 || !WIFEXITED(status))
                die("diff program failed");

        remove_tempfile();
}

void show_diff_empty(const struct cache_entry *ce, int reverse)
{
        struct diff_spec spec[2], *one, *two;

        memcpy(spec[0].u.sha1, ce->sha1, 20);
        spec[0].mode = ntohl(ce->ce_mode);
        spec[0].sha1_valid = spec[0].file_valid = 1;
        spec[1].file_valid = 0;

        if (reverse) {
                one = spec + 1; two = spec;
        } else {
                one = spec; two = one + 1;
        }

        run_external_diff(ce->name, one, two);
}

void show_differences(const struct cache_entry *ce, int reverse) 
{
        struct diff_spec spec[2], *one, *two;

        memcpy(spec[0].u.sha1, ce->sha1, 20);
        spec[0].mode = ntohl(ce->ce_mode);
        spec[0].sha1_valid = spec[0].file_valid = 1;

        spec[1].u.name = ce->name; /* the name we stated */
        spec[1].sha1_valid = 0;
        spec[1].file_valid = 1;

        if (reverse) {
                one = spec + 1; two = spec;
        } else {
                one = spec; two = one + 1;
        }

        run_external_diff(ce->name, one, two);
}