From 039ef019b9de7dd425b53abccc241da6999441a1 Mon Sep 17 00:00:00 2001
From: Joe Bryan <joemfb@gmail.com>
Date: Mon, 10 Aug 2020 12:56:54 -0700
Subject: [PATCH] ur: adds ur_walk_fore - higher-ordered pre-order noun
 traversal

---
 pkg/urbit/include/ur/hashcons.h |  7 +++++
 pkg/urbit/ur/hashcons.c         | 51 +++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/pkg/urbit/include/ur/hashcons.h b/pkg/urbit/include/ur/hashcons.h
index a6b83f388c..374c520754 100644
--- a/pkg/urbit/include/ur/hashcons.h
+++ b/pkg/urbit/include/ur/hashcons.h
@@ -163,3 +163,10 @@ ur_nvec_free(ur_nvec_t *v);
 
 void
 ur_nvec_init(ur_nvec_t *v, uint64_t size);
+
+void
+ur_walk_fore(ur_root_t *r,
+             ur_nref  ref,
+             void      *v,
+             void      (*atom)(ur_root_t*, ur_nref, void*),
+             ur_bool_t (*cell)(ur_root_t*, ur_nref, void*));
diff --git a/pkg/urbit/ur/hashcons.c b/pkg/urbit/ur/hashcons.c
index e07e839809..ced7cc37d1 100644
--- a/pkg/urbit/ur/hashcons.c
+++ b/pkg/urbit/ur/hashcons.c
@@ -821,3 +821,54 @@ ur_nvec_init(ur_nvec_t *v, uint64_t size)
   v->fill = 0;
   v->refs = calloc(size, sizeof(ur_nref));
 }
+
+void
+ur_walk_fore(ur_root_t *r,
+             ur_nref  ref,
+             void      *v,
+             void      (*atom)(ur_root_t*, ur_nref, void*),
+             ur_bool_t (*cell)(ur_root_t*, ur_nref, void*))
+{
+  uint64_t prev = 89, size = 144, fill = 0;
+  ur_nref *top, *don;
+
+  don  = malloc(size * sizeof(*don));
+  top  = don + ++fill;
+  *top = ref;
+
+  while ( top != don ) {
+    //  visit atom, pop stack
+    //
+    if ( !ur_deep(ref) ) {
+      atom(r, ref, v);
+      top--; fill--;
+    }
+    //  visit cell, pop stack if false
+    //
+    else if ( !cell(r, ref, v) ) {
+      top--; fill--;
+    }
+    //  push the tail, continue into the head
+    //
+    else {
+      *top = ur_tail(r, ref);
+
+      //  reallocate "stack" if full
+      //
+      if ( size == fill ) {
+        uint64_t next = prev + size;
+        don  = realloc(don, next * sizeof(*don));
+        top  = don + fill;
+        prev = size;
+        size = next;
+      }
+
+      top++; fill++;
+      *top = ur_head(r, ref);
+    }
+
+    ref = *top;
+  }
+
+  free(don);
+}