[PATCH v4 3/4] Utility function to seek in MIME trees in depth-first order.

[Austin Clements <amdragon@MIT.EDU>]

This function matches how we number parts for the --part argument to
show.  It will allow us to jump directly to the desired part, rather
than traversing the entire tree and carefully tracking whether or not
we're "in the zone".
---
 mime-node.c      |   27 +++++++++++++++++++++++++++
 notmuch-client.h |    5 +++++
 2 files changed, 32 insertions(+), 0 deletions(-)

diff --git a/mime-node.c b/mime-node.c
index fd8e754..1d32823 100644
--- a/mime-node.c
+++ b/mime-node.c
@@ -236,3 +236,30 @@ mime_node_child (const mime_node_t *parent, int child)
     }
     return _mime_node_create (parent, sub);
 }
+
+static mime_node_t *
+_mime_node_seek_dfs_walk (mime_node_t *node, int *n)
+{
+    mime_node_t *ret = NULL;
+    int i;
+
+    if (*n == 0)
+	return node;
+
+    *n -= 1;
+    for (i = 0; i < node->nchildren && !ret; i++) {
+	mime_node_t *child = mime_node_child (node, i);
+	ret = _mime_node_seek_dfs_walk (child, n);
+	if (!ret)
+	    talloc_free (child);
+    }
+    return ret;
+}
+
+mime_node_t *
+mime_node_seek_dfs (mime_node_t *node, int n)
+{
+    if (n < 0)
+	return NULL;
+    return _mime_node_seek_dfs_walk (node, &n);
+}
diff --git a/notmuch-client.h b/notmuch-client.h
index e23b51c..2f36519 100644
--- a/notmuch-client.h
+++ b/notmuch-client.h
@@ -324,5 +324,10 @@ mime_node_open (const void *ctx, notmuch_message_t *message,
 mime_node_t *
 mime_node_child (const mime_node_t *parent, int child);
 
+/* Return the nth child of node in a depth-first traversal.  If n is
+ * 0, returns node itself.  Returns NULL if there is no such part. */
+mime_node_t *
+mime_node_seek_dfs (mime_node_t *node, int n);
+
 #include "command-line-arguments.h"
 #endif
-- 
1.7.7.3