4 files changed, 55 insertions, 4 deletions

diff --git a/main.c b/main.c
index a326c2b..997dfe9 100644
--- a/main.c
+++ b/main.c
@@ -242,14 +242,46 @@ static struct tree_node* make_dir_tree(const struct options *opts, struct string
   return result;
 }
 
-static struct tree_node* make_dir_tree_root(const struct options *opts, const char *rootpath) {
-  int fd = open(rootpath, O_DIRECTORY);
+static struct tree_node* make_dir_tree_root(const struct options *opts, const char *rootpath, const char *treeroot) {
+  // If the first character is a /, keep it
+  size_t rootpath_len = strlen(rootpath);
+  while (rootpath_len > 1 && rootpath[rootpath_len - 1] == '/') rootpath_len--;
+
+  // But not in the tree root
+  size_t treeroot_len = treeroot != NULL ? strlen(treeroot) : 0;
+  while (treeroot_len > 0 && treeroot[treeroot_len - 1] == '/') treeroot_len--;
+
+  if (treeroot_len > 0 && treeroot[0] == '/') {
+    fprintf(stderr, "Tree root is not a relative path\n");
+    exit(1);
+  }
+
+  char *pathbuf = malloc(rootpath_len + 1 + treeroot_len + 1);
+  size_t pathbuf_len = rootpath_len;
+  strcpy(pathbuf, rootpath);
+  if (treeroot) {
+    pathbuf[pathbuf_len++] = '/';
+    strcpy(pathbuf + pathbuf_len, treeroot);
+    pathbuf_len += treeroot_len;
+  }
+
+  if (opts->debug) {
+    fprintf(stderr, "Expanding from path <%s>\n", pathbuf);
+  }
+
+  int fd = open(pathbuf, O_DIRECTORY);
   if (fd < 0) {
     perror("open");
     exit(1);
   }
 
-  struct string s = string_make("");
+  struct string s = string_make(treeroot_len > 0 ? pathbuf + rootpath_len+1 : "");
+
+  if (opts->debug) {
+    fprintf(stderr, "Virtual start path <%s>\n", string_read(s));
+  }
+
+  free(pathbuf);
 
   bool was_cache_dir = false;
   struct tree_node *tree = make_dir_tree(opts, &s, fd, &was_cache_dir);
@@ -300,7 +332,7 @@ int main(int argc, char **argv) {
     }
   }
 
-  struct tree_node *root = make_dir_tree_root(&opts, opts.rootpath);
+  struct tree_node *root = make_dir_tree_root(&opts, opts.rootpath, opts.treeroot);
 
   printf("files=%zu dirs=%zu size=%zu\n",
       root->total_files, root->total_dirs, root->total_size);
diff --git a/options.c b/options.c
index 2619602..6d43ccb 100644
--- a/options.c
+++ b/options.c
@@ -12,6 +12,7 @@ struct options parse_options(int argc, char **argv) {
   char **cachetags = malloc(cachetags_cap * sizeof(char*));
 
   char *rootpath = NULL;
+  char *treeroot = NULL;
 
   int print_depth = 1;
 
@@ -25,6 +26,20 @@ struct options parse_options(int argc, char **argv) {
       }
       rootpath = strdup(argv[i]);
 
+    } else if (strcmp(argv[i], "--treeroot") == 0) {
+      if (i + 1 >= argc) {
+        fprintf(stderr, "'--treeroot' takes an argument\n");
+        exit(1);
+      }
+
+      if (treeroot) {
+        fprintf(stderr, "Multiple tree roots given\n");
+        exit(1);
+      }
+      treeroot = strdup(argv[i+1]);
+
+      i++;
+
     } else if (strcmp(argv[i], "--exclude") == 0) {
       if (i + 1 >= argc) {
         fprintf(stderr, "'--exclude' takes an argument\n");
@@ -89,6 +104,7 @@ struct options parse_options(int argc, char **argv) {
     .ncachetags = cachetags_len,
     .cachetags = cachetags,
     .rootpath = rootpath,
+    .treeroot = treeroot,
     .print_depth = print_depth,
     .debug = debug,
   };
diff --git a/options.h b/options.h
index f015c33..402e492 100644
--- a/options.h
+++ b/options.h
@@ -13,6 +13,7 @@ struct options {
   char **cachetags;
 
   char *rootpath;
+  char *treeroot;  // relative within rootpath (or NULL)
 
   int print_depth;
 
diff --git a/string.h b/string.h
index ecb49b6..c2036a8 100644
--- a/string.h
+++ b/string.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <stddef.h>
+
 
 struct string {
   size_t len, cap;