/*
 * apply.c
 *
 * Copyright (C) Linus Torvalds, 2005
 *
 * This applies patches on top of some (arbitrary) version of the SCM.
 *
 * NOTE! It does all its work in the index file, and only cares about
 * the files in the working directory if you tell it to "merge" the
 * patch apply.
 *
 * Even when merging it always takes the source from the index, and
 * uses the working tree as a "branch" for a 3-way merge.
 */
#include <ctype.h>

#include "cache.h"

// We default to the merge behaviour, since that's what most people would
// expect
static int merge_patch = 1;
static const char apply_usage[] = "git-apply <patch>";

static int linenr = 1;

#define CHUNKSIZE (8192)

static void *read_patch_file(int fd, unsigned long *sizep)
{
        unsigned long size = 0, alloc = CHUNKSIZE;
        void *buffer = xmalloc(alloc);

        for (;;) {
                int nr = alloc - size;
                if (nr < 1024) {
                        alloc += CHUNKSIZE;
                        buffer = xrealloc(buffer, alloc);
                        nr = alloc - size;
                }
                nr = read(fd, buffer + size, nr);
                if (!nr)
                        break;
                if (nr < 0) {
                        if (errno == EAGAIN)
                                continue;
                        die("git-apply: read returned %s", strerror(errno));
                }
                size += nr;
        }
        *sizep = size;
        return buffer;
}

static unsigned long linelen(char *buffer, unsigned long size)
{
        unsigned long len = 0;
        while (size--) {
                len++;
                if (*buffer++ == '\n')
                        break;
        }
        return len;
}

static int match_word(const char *line, const char *match)
{
        for (;;) {
                char c = *match++;
                if (!c)
                        break;
                if (*line++ != c)
                        return 0;
        }
        return *line == ' ';
}

/* Verify that we recognize the lines following a git header */
static int parse_git_header(char *line, unsigned int size)
{
        unsigned long offset, len;

        for (offset = 0 ; size > 0 ; offset += len, size -= len, line += len, linenr++) {
                len = linelen(line, size);
                if (!len)
                        break;
                if (line[len-1] != '\n')
                        return -1;
                if (len < 4)
                        break;
                if (!memcmp(line, "@@ -", 4))
                        return offset;
                if (match_word(line, "new file mode"))
                        continue;
                if (match_word(line, "deleted file mode"))
                        continue;
                if (match_word(line, "copy"))
                        continue;
                if (match_word(line, "rename"))
                        continue;
                if (match_word(line, "similarity index"))
                        continue;
                break;
        }

        /* We want either a patch _or_ something real */
        return offset ? :-1;
}

static int parse_num(const char *line, int len, int offset, const char *expect, unsigned long *p)
{
        char *ptr;
        int digits, ex;

        if (offset < 0 || offset >= len)
                return -1;
        line += offset;
        len -= offset;

        if (!isdigit(*line))
                return -1;
        *p = strtoul(line, &ptr, 10);

        digits = ptr - line;

        offset += digits;
        line += digits;
        len -= digits;

        ex = strlen(expect);
        if (ex > len)
                return -1;
        if (memcmp(line, expect, ex))
                return -1;

        return offset + ex;
}

/*
 * Parse a unified diff fragment header of the
 * form "@@ -a,b +c,d @@"
 */
static int parse_fragment_header(char *line, int len, unsigned long *pos)
{
        int offset;

        if (!len || line[len-1] != '\n')
                return -1;

        /* Figure out the number of lines in a fragment */
        offset = parse_num(line, len, 4, ",", pos);
        offset = parse_num(line, len, offset, " +", pos+1);
        offset = parse_num(line, len, offset, ",", pos+2);
        offset = parse_num(line, len, offset, " @@", pos+3);

        return offset;
}

static int find_header(char *line, unsigned long size, int *hdrsize)
{
        unsigned long offset, len;

        for (offset = 0; size > 0; offset += len, size -= len, line += len, linenr++) {
                unsigned long nextlen;

                len = linelen(line, size);
                if (!len)
                        break;

                /* Testing this early allows us to take a few shortcuts.. */
                if (len < 6)
                        continue;

                /*
                 * Make sure we don't find any unconnected patch fragmants.
                 * That's a sign that we didn't find a header, and that a
                 * patch has become corrupted/broken up.
                 */
                if (!memcmp("@@ -", line, 4)) {
                        unsigned long pos[4];
                        if (parse_fragment_header(line, len, pos) < 0)
                                continue;
                        error("patch fragment without header at line %d: %.*s", linenr, len-1, line);
                }

                if (size < len + 6)
                        break;

                /*
                 * Git patch? It might not have a real patch, just a rename
                 * or mode change, so we handle that specially
                 */
                if (!memcmp("diff --git ", line, 11)) {
                        int git_hdr_len = parse_git_header(line + len, size - len);
                        if (git_hdr_len < 0)
                                continue;

                        *hdrsize = len + git_hdr_len;
                        return offset;
                }

                /** --- followed by +++ ? */
                if (memcmp("--- ", line,  4) || memcmp("+++ ", line + len, 4))
                        continue;

                /*
                 * We only accept unified patches, so we want it to
                 * at least have "@@ -a,b +c,d @@\n", which is 14 chars
                 * minimum
                 */
                nextlen = linelen(line + len, size - len);
                if (size < nextlen + 14 || memcmp("@@ -", line + len + nextlen, 4))
                        continue;

                /* Ok, we'll consider it a patch */
                *hdrsize = len + nextlen;
                linenr += 2;
                return offset;
        }
        return -1;
}

/*
 * Parse a unified diff. Note that this really needs
 * to parse each fragment separately, since the only
 * way to know the difference between a "---" that is
 * part of a patch, and a "---" that starts the next
 * patch is to look at the line counts..
 */
static int apply_fragment(char *line, unsigned long size)
{
        int len = linelen(line, size), offset;
        unsigned long pos[4], oldlines, newlines;

        offset = parse_fragment_header(line, len, pos);
        if (offset < 0)
                return -1;
        oldlines = pos[1];
        newlines = pos[3];

        /* Parse the thing.. */
        line += len;
        size -= len;
        linenr++;
        for (offset = len; size > 0; offset += len, size -= len, line += len, linenr++) {
                if (!oldlines && !newlines)
                        break;
                len = linelen(line, size);
                if (!len || line[len-1] != '\n')
                        return -1;
                switch (*line) {
                default:
                        return -1;
                case ' ':
                        oldlines--;
                        newlines--;
                        break;
                case '-':
                        oldlines--;
                        break;
                case '+':
                        newlines--;
                        break;
                }
        }
        return offset;
}

static int apply_single_patch(char *line, unsigned long size)
{
        unsigned long offset = 0;

        while (size > 4 && !memcmp(line, "@@ -", 4)) {
                int len = apply_fragment(line, size);
                if (len <= 0)
                        die("corrupt patch at line %d", linenr);

printf("applying fragment:\n%.*s\n\n", len, line);

                offset += len;
                line += len;
                size -= len;
        }
        return offset;
}

static int apply_chunk(char *buffer, unsigned long size)
{
        int hdrsize, patchsize;
        int offset = find_header(buffer, size, &hdrsize);
        char *header, *patch;

        if (offset < 0)
                return offset;
        header = buffer + offset;

printf("Found header:\n%.*s\n\n", hdrsize, header);

        patch = header + hdrsize;
        patchsize = apply_single_patch(patch, size - offset - hdrsize);

        return offset + hdrsize + patchsize;
}

static int apply_patch(int fd)
{
        unsigned long offset, size;
        char *buffer = read_patch_file(fd, &size);

        if (!buffer)
                return -1;
        offset = 0;
        while (size > 0) {
                int nr = apply_chunk(buffer + offset, size);
                if (nr < 0)
                        break;
                offset += nr;
                size -= nr;
        }
        free(buffer);
        return 0;
}

int main(int argc, char **argv)
{
        int i;

        if (read_cache() < 0)
                die("unable to read index file");

        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];
                int fd;

                if (!strcmp(arg, "-")) {
                        apply_patch(0);
                        continue;
                }
                if (!strcmp(arg, "--no-merge")) {
                        merge_patch = 0;
                        continue;
                }
                fd = open(arg, O_RDONLY);
                if (fd < 0)
                        usage(apply_usage);
                apply_patch(fd);
                close(fd);
        }
        return 0;
}