[PATCH] Introduce pathexpand: syntax-level chdir into the given cwd

[Alex Riesen <raa.lkml@gmail.com>]

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
---

This will be used by the following patch. I actually already have sent
this one in, as suggestion for some problem back then. It is a bit
generic, so it gets its own patch.

 cache.h |    2 ++
 path.c  |   59 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 0 deletions(-)

diff --git a/cache.h b/cache.h
index 054f106..25ce5da 100644
--- a/cache.h
+++ b/cache.h
@@ -383,6 +383,8 @@ static inline int is_absolute_path(const char *path)
 	return path[0] == '/';
 }
 const char *make_absolute_path(const char *path);
+/* Returns the analog of "cd path" from a directory "cwd" */
+extern char *pathexpand(const char *cwd, const char *path);
 
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern int sha1_object_info(const unsigned char *, unsigned long *);
diff --git a/path.c b/path.c
index 4260952..8231cd8 100644
--- a/path.c
+++ b/path.c
@@ -353,3 +353,62 @@ const char *make_absolute_path(const char *path)
 
 	return buf;
 }
+
+/*
+ * Returns the analog of "cd path" from a directory "cwd".
+ * The root is defined as empty path (instead of "/")
+ * An attempt to go past the root (with "..") leaves the path at root.
+ * The cwd is not expanded.
+ */
+char *pathexpand(const char *cwd, const char *path)
+{
+	static const char SEP[] = "/";
+	if (!*path) /* empty path -> "." (don't move) */
+		path = ".";
+	if (!cwd || !*cwd || *SEP == *path) /* no cwd, or path begins with "/" */
+		cwd = "";
+
+	while (*cwd && *SEP == *cwd)
+		++cwd;
+
+	size_t len = strlen(cwd);
+	char *out = malloc(len + 1 + strlen(path) + 1);
+	char *p = strcpy(out, cwd) + len;
+
+	for (; *path; ++path)
+	{
+		char *pl;
+		if (p > out && p[-1] != *SEP)
+			*p++ = *SEP;
+		pl = p;
+		while (*path && *SEP != *path)
+			*p++ = *path++;
+		*p = '\0';
+		/* ..."//"... */
+		if (p == pl)
+			; /* just ignore */
+		/* ..."/./"...  */
+		else if ( p - pl == 1 && '.' == *pl )
+			--p; /* just ignore */
+		/* ..."/../"...  */
+		else if ( p - pl == 2 && '.' == pl[0] && '.' == pl[1] )
+		{
+			/* drop the last element of the resulting path */
+			if (pl > out && --pl > out)
+				for (--pl; pl > out && *SEP != *pl; --pl)
+					;
+			p = pl > out ? ++pl: out;
+		}
+		/* ..."/path/"...  */
+		else if (*path)
+			*p++ = *path; /* just add the separator */
+
+		if (!*path)
+			break;
+	}
+	if (p > out+1 && *SEP == p[-1])
+		--p;
+	*p = '\0';
+	return out;
+}
+
-- 
1.5.4.rc0.86.g30f5