+#include "commit.h"
+
+static char *def = NULL;
+static int no_revs = 0;
+static int single_rev = 0;
+static int revs_only = 0;
+static int do_rev_argument = 1;
+static int output_revs = 0;
+
+#define NORMAL 0
+#define REVERSED 1
+static int show_type = NORMAL;
+
+static int get_extended_sha1(char *name, unsigned char *sha1);
+
+/*
+ * Some arguments are relevant "revision" arguments,
+ * others are about output format or other details.
+ * This sorts it all out.
+ */
+static int is_rev_argument(const char *arg)
+{
+ static const char *rev_args[] = {
+ "--max-count=",
+ "--max-age=",
+ "--min-age=",
+ "--merge-order",
+ NULL
+ };
+ const char **p = rev_args;
+
+ for (;;) {
+ const char *str = *p++;
+ int len;
+ if (!str)
+ return 0;
+ len = strlen(str);
+ if (!strncmp(arg, str, len))
+ return 1;
+ }
+}
+
+static void show_rev(int type, unsigned char *sha1)
+{
+ if (no_revs)
+ return;
+ output_revs++;
+ printf("%s%s\n", type == show_type ? "" : "^", sha1_to_hex(sha1));
+}
+
+static void show_rev_arg(char *rev)
+{
+ if (no_revs)
+ return;
+ puts(rev);
+}
+
+static void show_norev(char *norev)
+{
+ if (revs_only)
+ return;
+ puts(norev);
+}
+
+static void show_arg(char *arg)
+{
+ if (do_rev_argument && is_rev_argument(arg))
+ show_rev_arg(arg);
+ else
+ show_norev(arg);
+}
+
+static int get_parent(char *name, unsigned char *result, int idx)
+{
+ unsigned char sha1[20];
+ int ret = get_extended_sha1(name, sha1);
+ struct commit *commit;
+ struct commit_list *p;
+
+ if (ret)
+ return ret;
+ commit = lookup_commit_reference(sha1);
+ if (!commit)
+ return -1;
+ if (parse_commit(commit))
+ return -1;
+ p = commit->parents;
+ while (p) {
+ if (!--idx) {
+ memcpy(result, p->item->object.sha1, 20);
+ return 0;
+ }
+ p = p->next;
+ }
+ return -1;
+}
+
+/*
+ * This is like "get_sha1()", except it allows "sha1 expressions",
+ * notably "xyz^" for "parent of xyz"
+ */
+static int get_extended_sha1(char *name, unsigned char *sha1)
+{
+ int parent;
+ int len = strlen(name);
+
+ parent = 1;
+ if (len > 2 && name[len-1] >= '1' && name[len-1] <= '9') {
+ parent = name[len-1] - '0';
+ len--;
+ }
+ if (len > 1 && name[len-1] == '^') {
+ int ret;
+ name[len-1] = 0;
+ ret = get_parent(name, sha1, parent);
+ name[len-1] = '^';
+ if (!ret)
+ return 0;
+ }
+ return get_sha1(name, sha1);
+}
+
+static void show_default(void)
+{
+ char *s = def;
+
+ if (s) {
+ unsigned char sha1[20];
+
+ def = NULL;
+ if (!get_extended_sha1(s, sha1)) {
+ show_rev(NORMAL, sha1);
+ return;
+ }
+ show_arg(s);
+ }
+}