/* Per-turn file snapshots under <ws>/.agent/undo/<session>/<tag>/ so the
 * file changes of the latest turn can be rolled back (/undo). */
#ifndef _DEFAULT_SOURCE
#define _DEFAULT_SOURCE
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>

#include <unistd.h>

#include "cst_agent.h"
#include "cst_undo.h"
#include "cst_util.h"
#include "os_compat.h"

#define UNDO_FILE_MAX (8 * 1024 * 1024)

static int undo_root(const char *ws, const char *sid, char *out, size_t n) {
  int k = snprintf(out, n, "%s/.agent/undo/%s", ws, sid && sid[0] ? sid : "_");
  return k < 0 || (size_t)k >= n ? -1 : 0;
}

/* Workspace-relative, no "..", not absolute: undo may delete these paths. */
static int rel_ok(const char *rel) {
  if (!rel || !rel[0] || rel[0] == '/' || rel[0] == '\\' ||
      (rel[0] && rel[1] == ':'))
    return 0;
  const char *p = rel;
  while (*p) {
    size_t n = strcspn(p, "/\\");
    if (n == 2 && p[0] == '.' && p[1] == '.')
      return 0;
    p += n;
    while (*p == '/' || *p == '\\')
      p++;
  }
  return strchr(rel, '\n') == NULL && strchr(rel, '\t') == NULL;
}

static char *slurp(const char *p, size_t *len, size_t max) {
  *len = 0;
  FILE *fp = fopen(p, "rb");
  if (!fp)
    return NULL;
  fseek(fp, 0, SEEK_END);
  long sz = ftell(fp);
  if (sz < 0 || (size_t)sz > max) {
    fclose(fp);
    return NULL;
  }
  rewind(fp);
  char *b = malloc((size_t)sz + 1);
  if (!b) {
    fclose(fp);
    return NULL;
  }
  *len = fread(b, 1, (size_t)sz, fp);
  b[*len] = 0;
  fclose(fp);
  return b;
}

static int put(const char *p, const char *d, size_t n) {
  FILE *fp = fopen(p, "wb");
  if (!fp)
    return -1;
  int ok = n == 0 || fwrite(d, 1, n, fp) == n;
  return fclose(fp) == 0 && ok ? 0 : -1;
}

static int manifest_has(const char *mf, const char *rel) {
  FILE *fp = fopen(mf, "rb");
  if (!fp)
    return 0;
  char line[1400];
  int found = 0;
  while (!found && fgets(line, sizeof(line), fp)) {
    char *t2 = strchr(line, '\t');
    char *t3 = t2 ? strchr(t2 + 1, '\t') : NULL;
    if (!t3)
      continue;
    t3++;
    size_t n = strlen(t3);
    if (n && t3[n - 1] == '\n')
      t3[--n] = 0;
    found = strcmp(t3, rel) == 0;
  }
  fclose(fp);
  return found;
}

int cst_undo_snapshot(const char *ws, const char *sid, const char *tag,
                      const char *rel) {
  if (!ws || !tag || !rel_ok(rel))
    return -1;
  char full[OS_PATH_MAX];
  if (cst_sandbox_resolve(ws, rel, full, sizeof(full)) != 0) {
    /* parent missing: the file is new; record it as absent */
    snprintf(full, sizeof(full), "%s", "");
  }
  char dir[OS_PATH_MAX], mf[OS_PATH_MAX];
  if (undo_root(ws, sid, dir, sizeof(dir)) != 0)
    return -1;
  size_t dl = strlen(dir);
  int k = snprintf(dir + dl, sizeof(dir) - dl, "/%s", tag);
  if (k < 0 || (size_t)k >= sizeof(dir) - dl)
    return -1;
  if (os_mkdir_p(dir, 0755) != 0)
    return -1;
  k = snprintf(mf, sizeof(mf), "%s/MANIFEST", dir);
  if (k < 0 || (size_t)k >= sizeof(mf))
    return -1;
  if (manifest_has(mf, rel))
    return 0;
  int idx = 0;
  {
    FILE *fp = fopen(mf, "rb");
    if (fp) {
      int ch;
      while ((ch = fgetc(fp)) != EOF)
        idx += ch == '\n';
      fclose(fp);
    }
  }
  const char *state = "absent";
  struct stat st;
  if (full[0] && stat(full, &st) == 0 && S_ISREG(st.st_mode)) {
    size_t n = 0;
    char *data = slurp(full, &n, UNDO_FILE_MAX);
    if (!data) {
      state = "toolarge";
    } else {
      char bak[OS_PATH_MAX + 16];
      int bl = snprintf(bak, sizeof(bak), "%s/%d.bak", dir, idx);
      state = bl > 0 && (size_t)bl < sizeof(bak) && put(bak, data, n) == 0
                  ? "file"
                  : "toolarge";
      free(data);
    }
  }
  FILE *fp = fopen(mf, "ab");
  if (!fp)
    return -1;
  fprintf(fp, "%d\t%s\t%s\n", idx, state, rel);
  fclose(fp);
  return 0;
}

static int latest_tag(const char *root, char *tag, size_t n) {
  os_dir_t *d = os_dir_open(root);
  if (!d)
    return -1;
  const os_dirent_t *de;
  tag[0] = 0;
  while ((de = os_dir_next(d)) != NULL) {
    if (de->name[0] == '.')
      continue;
    if (strcmp(de->name, tag) > 0)
      snprintf(tag, n, "%s", de->name);
  }
  os_dir_close(d);
  return tag[0] ? 0 : -1;
}

static void rm_dir(const char *dir) {
  os_dir_t *d = os_dir_open(dir);
  if (d) {
    const os_dirent_t *de;
    while ((de = os_dir_next(d)) != NULL) {
      if (strcmp(de->name, ".") == 0 || strcmp(de->name, "..") == 0)
        continue;
      char p[OS_PATH_MAX];
      if (snprintf(p, sizeof(p), "%s/%s", dir, de->name) < (int)sizeof(p))
        unlink(p);
    }
    os_dir_close(d);
  }
  rmdir(dir);
}

int cst_undo_count(const char *ws, const char *sid) {
  char root[OS_PATH_MAX];
  if (undo_root(ws, sid, root, sizeof(root)) != 0)
    return 0;
  os_dir_t *d = os_dir_open(root);
  if (!d)
    return 0;
  int n = 0;
  const os_dirent_t *de;
  while ((de = os_dir_next(d)) != NULL)
    n += de->name[0] != '.';
  os_dir_close(d);
  return n;
}

int cst_undo_restore_last(const char *ws, const char *sid, char *report,
                          size_t report_sz) {
  report[0] = 0;
  char root[OS_PATH_MAX], tag[256], dir[OS_PATH_MAX], mf[OS_PATH_MAX];
  if (undo_root(ws, sid, root, sizeof(root)) != 0 ||
      latest_tag(root, tag, sizeof(tag)) != 0) {
    snprintf(report, report_sz, "nothing to undo");
    return -1;
  }
  if (snprintf(dir, sizeof(dir), "%s/%s", root, tag) >= (int)sizeof(dir) ||
      snprintf(mf, sizeof(mf), "%s/MANIFEST", dir) >= (int)sizeof(mf)) {
    snprintf(report, report_sz, "undo path too long");
    return -1;
  }
  FILE *fp = fopen(mf, "rb");
  cst_buf_t rep = {0};
  int restored = 0, failed = 0;
  if (fp) {
    char line[1400];
    while (fgets(line, sizeof(line), fp)) {
      size_t n = strlen(line);
      if (n && line[n - 1] == '\n')
        line[--n] = 0;
      char *t2 = strchr(line, '\t');
      char *t3 = t2 ? strchr(t2 + 1, '\t') : NULL;
      if (!t3)
        continue;
      *t2++ = 0;
      *t3++ = 0;
      const char *rel = t3, *state = t2;
      char full[OS_PATH_MAX];
      os_stat_t lst;
      if (!rel_ok(rel) || os_path_join(full, sizeof(full), ws, rel) != 0)
        continue;
      if (os_lstat(full, &lst) == 0 && lst.is_link) {
        cst_buf_printf(&rep, "skipped %s (now a symlink)\n", rel);
        failed++;
        continue;
      }
      if (strcmp(state, "file") == 0) {
        char bak[OS_PATH_MAX];
        if (snprintf(bak, sizeof(bak), "%s/%s.bak", dir, line) >=
            (int)sizeof(bak))
          bak[0] = 0;
        size_t len = 0;
        char *data = slurp(bak, &len, UNDO_FILE_MAX);
        char parent[OS_PATH_MAX];
        snprintf(parent, sizeof(parent), "%s", full);
        char *sl = strrchr(parent, '/');
        if (sl) {
          *sl = 0;
          os_mkdir_p(parent, 0755);
        }
        if (data && put(full, data, len) == 0) {
          cst_buf_printf(&rep, "restored %s\n", rel);
          restored++;
        } else {
          cst_buf_printf(&rep, "FAILED to restore %s\n", rel);
          failed++;
        }
        free(data);
      } else if (strcmp(state, "absent") == 0) {
        if (unlink(full) == 0) {
          cst_buf_printf(&rep, "removed %s (did not exist before)\n", rel);
          restored++;
        }
      } else {
        cst_buf_printf(&rep, "skipped %s (too large to snapshot)\n", rel);
        failed++;
      }
    }
    fclose(fp);
  }
  rm_dir(dir);
  if (!rep.buf)
    cst_buf_puts(&rep, "nothing changed\n");
  snprintf(report, report_sz, "%s", rep.buf);
  cst_buf_free(&rep);
  return failed && !restored ? -1 : 0;
}