Since 15ced75 (git-am foreign patch support: autodetect some patch
formats, 2009-05-27), git-am.sh is able to autodetect mbox, stgit and
mercurial patches through heuristics.

Re-implement support for autodetecting mbox/maildir files.

Helped-by: Eric Sunshine <sunsh...@sunshineco.com>
Signed-off-by: Paul Tan <pyoka...@gmail.com>
---
 builtin/am.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/builtin/am.c b/builtin/am.c
index e9a3687..7b97ea8 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -121,6 +121,96 @@ static void am_destroy(const struct am_state *state)
        strbuf_release(&sb);
 }
 
+/*
+ * Returns 1 if the file looks like a piece of email a-la RFC2822, 0 otherwise.
+ * We check this by grabbing all the non-indented lines and seeing if they look
+ * like they begin with valid header field names.
+ */
+static int is_email(const char *filename)
+{
+       struct strbuf sb = STRBUF_INIT;
+       FILE *fp = xfopen(filename, "r");
+       int ret = 1;
+
+       while (!strbuf_getline(&sb, fp, '\n')) {
+               const char *x;
+
+               strbuf_rtrim(&sb);
+
+               if (!sb.len)
+                       break; /* End of header */
+
+               /* Ignore indented folded lines */
+               if (*sb.buf == '\t' || *sb.buf == ' ')
+                       continue;
+
+               /* It's a header if it matches the regexp "^[!-9;-~]+:" */
+               for (x = sb.buf; *x; x++) {
+                       if (('!' <= *x && *x <= '9') || (';' <= *x && *x <= 
'~'))
+                               continue;
+                       if (*x == ':' && x != sb.buf)
+                               break;
+                       ret = 0;
+                       goto done;
+               }
+       }
+
+done:
+       fclose(fp);
+       strbuf_release(&sb);
+       return ret;
+}
+
+/**
+ * Attempts to detect the patch_format of the patches contained in `paths`,
+ * returning the PATCH_FORMAT_* enum value. Returns PATCH_FORMAT_UNKNOWN if
+ * detection fails.
+ */
+static int detect_patch_format(struct string_list *paths)
+{
+       enum patch_format ret = PATCH_FORMAT_UNKNOWN;
+       struct strbuf l1 = STRBUF_INIT;
+       struct strbuf l2 = STRBUF_INIT;
+       struct strbuf l3 = STRBUF_INIT;
+       FILE *fp;
+
+       /*
+        * We default to mbox format if input is from stdin and for directories
+        */
+       if (!paths->nr || !strcmp(paths->items->string, "-") ||
+           is_directory(paths->items->string)) {
+               ret = PATCH_FORMAT_MBOX;
+               goto done;
+       }
+
+       /*
+        * Otherwise, check the first 3 lines of the first patch, starting
+        * from the first non-blank line, to try to detect its format.
+        */
+       fp = xfopen(paths->items->string, "r");
+       while (!strbuf_getline(&l1, fp, '\n')) {
+               strbuf_trim(&l1);
+               if (l1.len)
+                       break;
+       }
+       strbuf_getline(&l2, fp, '\n');
+       strbuf_trim(&l2);
+       strbuf_getline(&l3, fp, '\n');
+       strbuf_trim(&l3);
+       fclose(fp);
+
+       if (starts_with(l1.buf, "From ") || starts_with(l1.buf, "From: "))
+               ret = PATCH_FORMAT_MBOX;
+       else if (l1.len && l2.len && l3.len && is_email(paths->items->string))
+               ret = PATCH_FORMAT_MBOX;
+
+done:
+       strbuf_release(&l1);
+       strbuf_release(&l2);
+       strbuf_release(&l3);
+       return ret;
+}
+
 /**
  * Splits out individual patches from `paths`, where each path is either a mbox
  * file or a Maildir. Return 0 on success, -1 on failure.
@@ -177,6 +267,14 @@ static int split_patches(struct am_state *state, enum 
patch_format patch_format,
 static void am_setup(struct am_state *state, enum patch_format patch_format,
                struct string_list *paths)
 {
+       if (!patch_format)
+               patch_format = detect_patch_format(paths);
+
+       if (!patch_format) {
+               fprintf_ln(stderr, _("Patch format detection failed."));
+               exit(128);
+       }
+
        if (mkdir(state->dir.buf, 0777) < 0 && errno != EEXIST)
                die_errno(_("failed to create directory '%s'"), state->dir.buf);
 
-- 
2.1.4

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