diff --git a/bolt_checkin_notes.txt b/bolt_checkin_notes.txt
index 6b1145206d..5aebbfab84 100644
--- a/bolt_checkin_notes.txt
+++ b/bolt_checkin_notes.txt
@@ -139,3 +139,10 @@ David  Feb 22 2008
             bolt.inc
         ops/
             bolt_map.php
+
+David  June 26 2008
+    - (a bunch of checkins recently)
+        Added basic Unit Comparison feature, including
+        - snapshots (makes it fast)
+        - filtering and breakdown
+        Also revisited Maps, and started redoing them along the same lines
diff --git a/html/inc/bolt_db.inc b/html/inc/bolt_db.inc
index 478b7d30dd..7238518bb3 100644
--- a/html/inc/bolt_db.inc
+++ b/html/inc/bolt_db.inc
@@ -181,7 +181,7 @@ class BoltResult {
     }
     static function enum($clause) {
         $db = BoltDb::get();
-        return $db->enum('bolt_view', 'BoltResult', $clause);
+        return $db->enum('bolt_result', 'BoltResult', $clause);
     }
 }
 
diff --git a/html/inc/bolt_util.inc b/html/inc/bolt_util.inc
index 42fe8cf409..88d1edec09 100644
--- a/html/inc/bolt_util.inc
+++ b/html/inc/bolt_util.inc
@@ -113,7 +113,7 @@ function request_info($user, $course) {
 ////// stuff related to snapshots
 
 function compare_snapshot_filename($course_id, $select_name, $xset_name) {
-    return "../bolt_snap/compare_snapshot_".$course_id."_".$select_name."_".$xset_name.".json";
+    return "../bolt_snap/compare_snapshot_".$course_id."_".$select_name."_".$xset_name;
 }
 
 // a "snapshot" is a condensed representation of the results
@@ -186,7 +186,7 @@ function write_compare_snapshot($course_id, $select_name, $xset_name, $dur) {
     $s->recs = $a;
     $s->dur = $dur;
     $s->time = $now;
-    fwrite($f, json_encode($s));
+    fwrite($f, serialize($s));
     fclose($f);
     return $s;
 }
@@ -197,12 +197,12 @@ function read_compare_snapshot($course_id, $select_name, $xset_name) {
     if (!$f) return null;
     $x = fread($f, filesize($filename));
     fclose($f);
-    return json_decode($x);
+    return unserialize($x);
 }
 
 
 function map_snapshot_filename($course_id) {
-    return "../bolt_snap/map_snapshot_".$course_id.".json";
+    return "../bolt_snap/map_snapshot_".$course_id;
 }
 
 // a "map snapshot" is:
@@ -216,12 +216,13 @@ function write_map_snapshot($course_id, $dur) {
     $start = $now - $dur*86400;
 
     $views = array();
+    $results = array();
     $xset_results = array();
     $users = array();
 
-    $vs = BoltView::enum("course_id=$course_id and create_time>$start");
+    $vs = BoltView::enum("course_id=$course_id and start_time>$start");
     foreach ($vs as $v) {
-        if (array_key_exists($v->item_name), $views) {
+        if (array_key_exists($v->item_name, $views)) {
             $x = $views[$v->item_name];
             $x[] = $v;
             $views[$v->item_name] = $x;
@@ -237,7 +238,7 @@ function write_map_snapshot($course_id, $dur) {
 
     $rs = BoltResult::enum("course_id=$course_id and create_time>$start");
     foreach ($rs as $r) {
-        if (array_key_exists($r->item_name), $results) {
+        if (array_key_exists($r->item_name, $results)) {
             $x = $results[$r->item_name];
             $x[] = $r;
             $results[$r->item_name] = $x;
@@ -253,15 +254,15 @@ function write_map_snapshot($course_id, $dur) {
 
     $xrs = BoltXsetResult::enum("course_id=$course_id and create_time>$start");
     foreach ($xrs as $xr) {
-        if (array_key_exists($xr->item_name), $xset_results) {
-            $x = $xset_results[$xr->item_name];
+        if (array_key_exists($xr->name, $xset_results)) {
+            $x = $xset_results[$xr->name];
             $x[] = $xr;
-            $xset_results[$xr->item_name] = $x;
+            $xset_results[$xr->name] = $x;
         } else {
-            $xset_results[$xr->item_name] = array($xr);
+            $xset_results[$xr->name] = array($xr);
         }
         if (!array_key_exists($xr->user_id, $users)) {
-            $user = BoincUser::lookup_id($r->user_id);
+            $user = BoincUser::lookup_id($xr->user_id);
             BoltUser::lookup($user);
             $users[$v->user_id] = $user;
         }
@@ -277,18 +278,19 @@ function write_map_snapshot($course_id, $dur) {
 
     $filename = map_snapshot_filename($course_id);
     $f = fopen($filename, "w");
-    fwrite($f, json_encode($y));
+    fwrite($f, serialize($y));
     fclose($f);
+
     return $y;
 }
 
-function read_map_snapshot() {
-    $filename = map_snapshot_filename($course_id, $select_name, $xset_name);
+function read_map_snapshot($course_id) {
+    $filename = map_snapshot_filename($course_id);
     $f = @fopen($filename, "r");
     if (!$f) return null;
     $x = fread($f, filesize($filename));
     fclose($f);
-    return json_decode($x);
+    return unserialize($x);
 }
 
 ////// Statistics
diff --git a/html/ops/bolt_map.php b/html/ops/bolt_map.php
index 927ac808fa..eec9d3d1fc 100644
--- a/html/ops/bolt_map.php
+++ b/html/ops/bolt_map.php
@@ -10,9 +10,10 @@
 //      show a map;
 //      show form to set or change filter or breakdown
 
-require_once("../util.inc");
+require_once("../inc/util.inc");
 require_once("../inc/bolt_db.inc");
 require_once("../inc/bolt_cat.inc");
+require_once("../inc/bolt_util.inc");
 require_once("../inc/bolt.inc");
 
 function show_snap_form() {
@@ -46,25 +47,72 @@ function show_snap_form() {
 }
 
 function snap_action() {
-     global $course_id;
-     $dur = get_int('dur');
-     $s = write_map_snapshot($course_id, $dur);
-     map_aux($select_name, $xset_name, $s);
+    global $course_id;
+    global $top_unit;
 
+    $dur = get_int('dur');
+    $s = write_map_snapshot($course_id, $dur);
+    show_map();
 }
 
-function map_aux($snap) {
+function show_unit($snap, $unit) {
+    $class = get_class($unit);
+    echo "<li> $unit->name ($class); ";
+    if ($unit->is_item) {
+        if (array_key_exists($unit->name, $snap->views)) {
+            $n = count($snap->views[$unit->name]);
+        } else {
+            $n = 0;
+        }
+        echo "$n views";
+    }
+    if ($class == "BoltExercise") {
+        if (array_key_exists($unit->name, $snap->results)) {
+            $rs = $snap->results[$unit->name];
+            $sum = 0;
+            $n = count($rs);
+            foreach ($rs as $r) {
+                $sum += $r->score;
+            }
+            $avg = $sum/$n;
+            echo " avg score: $avg ($n)";
+        }
+    }
+    if ($class == "BoltExerciseSet") {
+        if (array_key_exists($unit->name, $snap->xset_results)) {
+            $xrs = $snap->xset_results[$unit->name];
+            $sum = 0;
+            $n = count($xrs);
+            foreach ($xrs as $xr) {
+                $sum += $xr->score;
+            }
+            $avg = $sum/$n;
+            echo " avg score: $avg ($n)";
+        }
+    }
+    echo "\n";
 }
 
-function show_map($unit, $level) {
-    for ($i=0; $i<$level; $i++) echo '  ';
-    echo "$unit->name: $unit->nviews\n";
+function show_unit_recurse($snap, $unit) {
+    show_unit($snap, $unit);
     if ($unit->is_item) return;
     foreach ($unit->units as $u) {
-        show_map($u, $level+1);
+        echo "<ul>\n";
+        show_unit_recurse($snap, $u);
+        echo "</ul>\n";
     }
 }
 
+function show_map() {
+    global $course_id;
+    global $top_unit;
+
+    page_head("Course map");
+    $snap = read_map_snapshot($course_id);
+    show_unit_recurse($snap, $top_unit);
+    page_tail();
+}
+
 $course_id = get_int('course_id');
 $course = BoltCourse::lookup_id($course_id);
 if (!$course) error_page("no course");
@@ -79,7 +127,7 @@ case "snap_action":
     snap_action();
     break;
 case "map":
-    show_map($top_unit, 0);
+    show_map();
     break;
 default:
     error_page("Unknown action $action");