diff --git a/Makefile b/Makefile
index a06ff0cdb2..8ddcd095d5 100644
--- a/Makefile
+++ b/Makefile
@@ -4,7 +4,7 @@ CC=gcc
 CXX=g++
 CFLAGS=-std=c99
 CPPFLAGS=-Wall -Wextra -pedantic -g -DUPB_UNALIGNED_READS_OK -fomit-frame-pointer
-OBJ=upb_parse.o upb_table.o upb_msg.o upb_enum.o upb_context.o descriptor.o
+OBJ=upb_parse.o upb_table.o upb_msg.o upb_enum.o upb_context.o upb_string.o descriptor.o
 all: $(OBJ) test_table tests upbc
 clean:
 	rm -f *.o test_table tests
diff --git a/test_table.cc b/test_table.cc
index 947a7a4860..0337eaa399 100644
--- a/test_table.cc
+++ b/test_table.cc
@@ -247,7 +247,7 @@ int main()
   keys.push_back("google.protobuf.UninterpretedOption.NamePart");
 
   test_strtable(keys, 18);
-  return 0;
+
   int32_t *keys1 = get_contiguous_keys(8);
   printf("Contiguous 1-8 ====\n");
   test_inttable(keys1, 8);
diff --git a/upb_context.c b/upb_context.c
index e92f9e31a1..d83906c7e1 100644
--- a/upb_context.c
+++ b/upb_context.c
@@ -11,6 +11,7 @@
 #include "upb_enum.h"
 #include "upb_msg.h"
 
+/* Search for a character in a string, in reverse. */
 static int memrchr(char *data, char c, size_t len)
 {
   int off = len-1;
@@ -70,6 +71,8 @@ struct upb_symtab_entry *upb_context_lookup(struct upb_context *c,
   return upb_strtable_lookup(&c->symtab, symbol);
 }
 
+/* Given a symbol and the base symbol inside which it is defined, find the
+ * symbol's definition in t. */
 static struct upb_symtab_entry *resolve(struct upb_strtable *t,
                                         struct upb_string *base,
                                         struct upb_string *symbol)
@@ -103,6 +106,7 @@ static struct upb_symtab_entry *resolve(struct upb_strtable *t,
   }
 }
 
+/* Tries to resolve a symbol in two different tables. */
 union upb_symbol_ref resolve2(struct upb_strtable *t1, struct upb_strtable *t2,
                               struct upb_string *base, struct upb_string *sym,
                                  enum upb_symbol_type expected_type) {
@@ -121,8 +125,9 @@ struct upb_symtab_entry *upb_context_resolve(struct upb_context *c,
   return resolve(&c->symtab, base, symbol);
 }
 
-/* join("Foo.Bar", "Baz") -> "Foo.Bar.Baz"
- * join("", "Baz") -> "Baz"
+/* Joins strings together, for example:
+ *   join("Foo.Bar", "Baz") -> "Foo.Bar.Baz"
+ *   join("", "Baz") -> "Baz"
  * Caller owns the returned string and must free it. */
 static struct upb_string join(struct upb_string *base, struct upb_string *name) {
   size_t len = base->byte_len + name->byte_len;
@@ -192,7 +197,7 @@ static bool insert_message(struct upb_strtable *t,
   upb_strtable_insert(t, &e.e);
 
   /* Add nested messages and enums. */
-  //if(d->set_flags.has.nested_type)
+  //if(d->set_flags.has.nested_type)  (re-enable when protoc sets)
   if(d->nested_type)
     for(unsigned int i = 0; i < d->nested_type->len; i++)
       if(!insert_message(t, d->nested_type->elements[i], &fqname))
@@ -268,7 +273,10 @@ bool upb_context_parsefds(struct upb_context *c, struct upb_string *fds_str) {
   google_protobuf_FileDescriptorSet *fds =
       upb_alloc_and_parse(c->fds_msg, fds_str, true);
   if(!fds) return false;
+
   if(fds->set_flags.has.file) {
+    /* Insert new symbols into a temporary table until we have verified that
+     * the descriptor is valid. */
     struct upb_strtable tmp;
     upb_strtable_init(&tmp, 0, sizeof(struct upb_symtab_entry));
     for(uint32_t i = 0; i < fds->file->len; i++) {
@@ -284,10 +292,12 @@ bool upb_context_parsefds(struct upb_context *c, struct upb_string *fds_str) {
       upb_strtable_insert(&c->symtab, &e->e);
     upb_strtable_free(&tmp);
   }
+
+  /* We own fds now, need to keep a ref so we can free it later. */
   if(c->fds_size == c->fds_len) {
     c->fds_size *= 2;
     c->fds = realloc(c->fds, c->fds_size);
   }
-  c->fds[c->fds_len++] = fds;  /* Need to keep a ref since we own it. */
+  c->fds[c->fds_len++] = fds;
   return true;
 }
diff --git a/upb_msg.c b/upb_msg.c
index 48804e76eb..c9fa260b82 100644
--- a/upb_msg.c
+++ b/upb_msg.c
@@ -17,6 +17,7 @@ static int div_round_up(int numerator, int denominator) {
   return numerator > 0 ? (numerator - 1) / denominator + 1 : 0;
 }
 
+/* Callback for sorting fields. */
 static int compare_fields(const void *e1, const void *e2) {
   const google_protobuf_FieldDescriptorProto *fd1 = *(void**)e1;
   const google_protobuf_FieldDescriptorProto *fd2 = *(void**)e2;
@@ -243,6 +244,7 @@ void upb_msg_reuse_submsg(void **msg, struct upb_msg *m)
 
 /* Serialization/Deserialization.  ********************************************/
 
+/* We use this as our "user_data" for each frame of the parsing stack. */
 struct parse_frame_data {
   struct upb_msg *m;
   void *data;
@@ -267,6 +269,8 @@ static upb_field_type_t tag_cb(struct upb_parse_state *s, struct upb_tag *tag,
   return f->type;
 }
 
+/* Returns a pointer to where the next value for field "f" should be stored,
+ * taking into account whether f is an array that may need to be reallocatd. */
 static union upb_value_ptr get_value_ptr(void *data, struct upb_msg_field *f)
 {
   union upb_value_ptr p = upb_msg_getptr(data, f);
@@ -315,6 +319,7 @@ static void submsg_start_cb(struct upb_parse_state *_s, void *user_field_desc)
   struct upb_msg_parse_state *s = (void*)_s;
   struct upb_msg_field *f = user_field_desc;
   struct parse_frame_data *frame = (void*)&s->s.top->user_data;
+  // TODO: find a non-hacky way to get a pointer to the old frame.
   struct parse_frame_data *oldframe = (void*)((char*)s->s.top - s->s.udata_size);
   union upb_value_ptr p = get_value_ptr(oldframe->data, f);
   upb_msg_reuse_submsg(p.message, f->ref.msg);
@@ -363,74 +368,3 @@ void *upb_alloc_and_parse(struct upb_msg *m, struct upb_string *str, bool byref)
   }
 }
 
-static void dump_value(union upb_value_ptr p, upb_field_type_t type, FILE *stream)
-{
-  switch(type) {
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_DOUBLE:
-      fprintf(stream, "%f", *p._double); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_FLOAT:
-      fprintf(stream, "%f", *p._float); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_FIXED64:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_UINT64:
-      fprintf(stream, "%" PRIu64, *p.uint64); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_FIXED32:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_UINT32:
-      fprintf(stream, "%" PRIu32, *p.uint32); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_BOOL:
-      if(*p._bool) fputs("true", stream);
-      else fputs("false", stream);
-      break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_GROUP:
-      break; /* can't actually be stored */
-
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_MESSAGE:
-      fprintf(stream, "%p", (void*)*p.message); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_STRING:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_BYTES:
-      if(*p.string)
-        fprintf(stream, "\"" UPB_STRFMT "\"", UPB_STRARG(**p.string));
-      else
-        fputs("(null)", stream);
-      break;
-
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_ENUM:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_INT32:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_SFIXED32:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_SINT32:
-      fprintf(stream, "%" PRId32, *p.int32); break;
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_SFIXED64:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_SINT64:
-    case GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_TYPE_INT64:
-      fprintf(stream, "%" PRId64, *p.int64); break;
-  }
-}
-
-void upb_msg_print(void *data, struct upb_msg *m, FILE *stream)
-{
-  fprintf(stream, "Message <" UPB_STRFMT ">\n", UPB_STRARG(*m->descriptor->name));
-  for(uint32_t i = 0; i < m->num_fields; i++) {
-    struct upb_msg_field *f = &m->fields[i];
-    google_protobuf_FieldDescriptorProto *fd = m->field_descriptors[i];
-    fprintf(stream, "  " UPB_STRFMT, UPB_STRARG(*fd->name));
-
-    if(upb_msg_is_set(data, f)) fputs(" (set): ", stream);
-    else fputs(" (NOT set): ", stream);
-
-    union upb_value_ptr p = upb_msg_getptr(data, f);
-    if(f->label == GOOGLE_PROTOBUF_FIELDDESCRIPTORPROTO_LABEL_REPEATED) {
-      if(*p.array) {
-        fputc('[', stream);
-        for(uint32_t j = 0; j < (*p.array)->len; j++) {
-          dump_value(upb_array_getelementptr(*p.array, j, f->type), f->type, stream);
-          if(j != (*p.array)->len - 1) fputs(", ", stream);
-        }
-        fputs("]\n", stream);
-      } else {
-        fputs("<null>\n", stream);
-      }
-    } else {
-      dump_value(p, f->type, stream);
-      fputc('\n', stream);
-    }
-  }
-}
diff --git a/upb_string.c b/upb_string.c
new file mode 100644
index 0000000000..733bafe361
--- /dev/null
+++ b/upb_string.c
@@ -0,0 +1,24 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2009 Joshua Haberman.  See LICENSE for details.
+ */
+
+#include "upb_string.h"
+
+bool upb_strreadfile(const char *filename, struct upb_string *data) {
+  FILE *f = fopen(filename, "rb");
+  if(!f) return false;
+  if(fseek(f, 0, SEEK_END) != 0) return false;
+  long size = ftell(f);
+  if(size < 0) return false;
+  if(fseek(f, 0, SEEK_SET) != 0) return false;
+  data->ptr = (char*)malloc(size);
+  data->byte_len = size;
+  if(fread(data->ptr, size, 1, f) != 1) {
+    free(data->ptr);
+    return false;
+  }
+  fclose(f);
+  return true;
+}
diff --git a/upb_string.h b/upb_string.h
index 2520caa714..44bc653bf9 100644
--- a/upb_string.h
+++ b/upb_string.h
@@ -1,6 +1,8 @@
 /*
  * upb - a minimalist implementation of protocol buffers.
  *
+ * Copyright (c) 2009 Joshua Haberman.  See LICENSE for details.
+
  * Defines a delimited (as opposed to null-terminated) string type and some
  * library functions for manipulating them.
  *
@@ -16,8 +18,6 @@
  * string data.  With NULL-termination I would be forced to write a NULL
  * into the middle of the protobuf's data, which is less than ideal and in
  * some cases not practical or possible.
- *
- * Copyright (c) 2009 Joshua Haberman.  See LICENSE for details.
  */
 
 #ifndef UPB_STRING_H_
@@ -29,6 +29,7 @@ extern "C" {
 
 #include <stdlib.h>
 #include <string.h>
+#include "upb.h"
 
 struct upb_string {
   /* We expect the data to be 8-bit clean (uint8_t), but char* is such an
@@ -59,24 +60,17 @@ INLINE void upb_strfree(struct upb_string s) {
   free(s.ptr);
 }
 
-INLINE bool upb_strreadfile(const char *filename, struct upb_string *data) {
-  FILE *f = fopen(filename, "rb");
-  if(!f) return false;
-  if(fseek(f, 0, SEEK_END) != 0) return false;
-  long size = ftell(f);
-  if(size < 0) return false;
-  if(fseek(f, 0, SEEK_SET) != 0) return false;
-  data->ptr = (char*)malloc(size);
-  data->byte_len = size;
-  if(fread(data->ptr, size, 1, f) != 1) {
-    free(data->ptr);
-    return false;
-  }
-  fclose(f);
-  return true;
-}
+/* Reads an entire file into a newly-allocated string. */
+bool upb_strreadfile(const char *filename, struct upb_string *data);
 
+/* Allows defining upb_strings as literals, ie:
+ *   struct upb_string str = UPB_STRLIT("Hello, World!\n");
+ */
 #define UPB_STRLIT(strlit) {.ptr=strlit, .byte_len=sizeof(strlit)-1}
+
+/* Allows using upb_strings in printf, ie:
+ *   struct upb_string str = UPB_STRLIT("Hello, World!\n");
+ *   printf("String is: " UPB_STRFMT, UPB_STRARG(str)); */
 #define UPB_STRARG(str) (str).byte_len, (str).ptr
 #define UPB_STRFMT "%.*s"
 
diff --git a/upb_table.c b/upb_table.c
index d53ab1c33e..bad0b23a46 100644
--- a/upb_table.c
+++ b/upb_table.c
@@ -77,8 +77,12 @@ static uint32_t empty_intbucket(struct upb_inttable *table)
   return 0;
 }
 
+/* The insert routines have a lot more code duplication between int/string
+ * variants than I would like, but there's just a bit too much that varies to
+ * parameterize them. */
 static void intinsert(struct upb_inttable *t, struct upb_inttable_entry *e)
 {
+  assert(upb_inttable_lookup(t, e->key, t->t.entry_size) == NULL);
   uint32_t bucket = upb_inttable_bucket(t, e->key);
   struct upb_inttable_entry *table_e = intent(t, bucket);
   if(table_e->key != EMPTYENT) {  /* Collision. */
@@ -110,6 +114,7 @@ static void intinsert(struct upb_inttable *t, struct upb_inttable_entry *e)
   }
   memcpy(table_e, e, t->t.entry_size);
   table_e->next = UPB_END_OF_CHAIN;
+  assert(upb_inttable_lookup(t, e->key, t->t.entry_size) == table_e);
 }
 
 void upb_inttable_insert(struct upb_inttable *t, struct upb_inttable_entry *e)
@@ -141,6 +146,7 @@ static uint32_t empty_strbucket(struct upb_strtable *table)
 
 static void strinsert(struct upb_strtable *t, struct upb_strtable_entry *e)
 {
+  assert(upb_strtable_lookup(t, &e->key) == NULL);
   uint32_t bucket = strtable_bucket(t, &e->key);
   struct upb_strtable_entry *table_e = strent(t, bucket);
   if(table_e->key.byte_len != 0) {  /* Collision. */
diff --git a/upb_table.h b/upb_table.h
index a5da0f2c6a..8eec03c6a6 100644
--- a/upb_table.h
+++ b/upb_table.h
@@ -23,6 +23,7 @@
 extern "C" {
 #endif
 
+/* Note: the key cannot be zero!  Zero is used by the implementation. */
 typedef uint32_t upb_inttable_key_t;
 
 #define UPB_END_OF_CHAIN (uint32_t)0