diff --git a/builder/index.html b/builder/index.html
index 066fa2a..2d20f35 100644
--- a/builder/index.html
+++ b/builder/index.html
@@ -429,7 +429,7 @@
                                             </div>
                                             <div class = "col-3 py-2">
                                                 <button class = "col-auto button rounded scaled-font fw-bold text-light dark-5" id = "toggle-atree" onclick = "toggle_tab('atree-dropdown'); toggleButton('toggle-atree')">
-                                                    Show Ability Tree
+                                                    Show Tree
                                                 </button>
                                             </div>
                                             <div class = "col-3 py-2">
@@ -618,11 +618,11 @@
                                     </div>
                                     <div class = "col dark-6 rounded-bottom my-3 my-xl-1" id = "atree-dropdown" style = "display:none;">
                                         <div class="row row-cols-1 row-cols-xl-2">
-                                            <div class="col border border-semi-light rounded dark-9 hide-scroll" id="atree-ui" style="height: 80vh; overflow-y: auto;">
+                                            <div class="col border border-semi-light rounded dark-9 hide-scroll" id="atree-ui" style="height: 90vh; overflow-y: auto;">
 
                                             </div>
-                                            <div class="col mx-auto" style="height: 80vh; overflow-y: auto;" id="atree-rhs">
-                                            <div class="col mx-auto" style="height: 4em; overflow-y: auto;" id="atree-header">
+                                            <div class="col mx-auto" style="height: 90vh; overflow-y: auto;" id="atree-rhs">
+                                            <div class="col mx-auto" style="height: 2em; overflow-y: auto;" id="atree-header">
                                             </div>
                                             <div class="col mx-auto" style="overflow-y: auto;" id="atree-active">
                                             </div>
@@ -1336,7 +1336,7 @@
             <div class="col-12 dark-5 scaled-font">
                 <footer class="text-center">
                     <div id="header2">
-                        <p>Made by <b class = "hppeng">hppeng</b> and <b class = "ferricles">ferricles</b> with <a href = "../atlas" target = "_blank" class = "atlas link">Atlas Inc</a> (JavaScript required to function, nothing works without js)</p>
+                        <p>Made by <b class = "hppeng">hppeng</b>, <b class = "ferricles">ferricles</b>, and <b>reschan</b> with <a href = "../atlas" target = "_blank" class = "atlas link">Atlas Inc</a> (JavaScript required to function, nothing works without js)</p>
                         <p>Hard refresh the page (Ctrl+Shift+R on windows/chrome) if it isn't updating correctly.</p>
                     </div>
                     <div id="credits">
diff --git a/crafter/index.html b/crafter/index.html
index 0359611..65c6cdc 100644
--- a/crafter/index.html
+++ b/crafter/index.html
@@ -282,7 +282,7 @@
             <div class="col dark-5 scaled-font">
                 <footer class="text-center">
                     <div id="header2">
-                        <p>Made by <b class = "hppeng">hppeng</b> and <b class = "ferricles">ferricles</b> with <a href = "../atlas" target = "_blank" class = "atlas link">Atlas Inc</a> (JavaScript required to function, nothing works without js)</p>
+                        <p>Made by <b class = "hppeng">hppeng</b>, <b class = "ferricles">ferricles</b>, and <b>reschan</b> with <a href = "../atlas" target = "_blank" class = "atlas link">Atlas Inc</a> (JavaScript required to function, nothing works without js)</p>
                         <p>Hard refresh the page (Ctrl+Shift+R on windows/chrome) if it isn't updating correctly.</p>
                     </div>
                     <div id="credits">
diff --git a/credits.txt b/credits.txt
new file mode 100644
index 0000000..36a1694
--- /dev/null
+++ b/credits.txt
@@ -0,0 +1,20 @@
+
+Theme, formatting, and overall inspiration: Wynndata (Dukio)
+    - https://wynndata.tk
+
+The game, of course
+    - wynncraft.com
+
+Additional Contributors, in no particular order:
+ - Kiocifer (Icons!)
+ - IncinerateMe (helping transition to 1.20.3 / CI helper)
+ - puppy (wynn2 ability tree help)
+ - SockMower (ability tree encode/decode optimization)
+ - ITechnically (coding emotional support / misc)
+ - touhoku (best IM)
+ - HeyZeer0 (huge help in getting our damage formulas right)
+ - Lennon (Skill point formula reversing)
+ - Phanta (WynnAtlas custom expression parser / item search)
+ - nbcss (Crafted Item mechanics reverse engineering)
+ - dr_carlos (Hiding UI elements properly, fade animations, proper error handling)
+ - Atlas Inc discord (feedback, ideas, damage calc, etc)
diff --git a/js/atree.js b/js/atree.js
index 00c1391..a91f7e4 100644
--- a/js/atree.js
+++ b/js/atree.js
@@ -168,15 +168,23 @@ const atree_node = new (class extends ComputeNode {
  * Signature: AbilityTreeRenderNode(atree: ATree) => RenderedATree ( Map[id, RenderedATNode] )
  */
 const atree_render = new (class extends ComputeNode {
-    constructor() { super('builder-atree-render'); this.fail_cb = true; }
+    constructor() {
+        super('builder-atree-render');
+        this.fail_cb = true;
+        this.UI_elem = document.getElementById("atree-ui");
+        this.list_elem = document.getElementById("atree-header");
+    }
 
     compute_func(input_map) {
         if (input_map.size !== 1) { throw "AbilityTreeRenderNode accepts exactly one input (atree)"; }
         const [atree] = input_map.values();  // Extract values, pattern match it into size one list and bind to first element
         
         //for some reason we have to cast to string 
+        this.list_elem.innerHTML = ""; //reset all atree actives - should be done in a more general way later
+        this.UI_elem.innerHTML = ""; //reset the atree in the DOM
+
         let ret = null;
-        if (atree) { ret = render_AT(document.getElementById("atree-ui"), document.getElementById("atree-header"), atree); }
+        if (atree) { ret = render_AT(this.UI_elem, this.list_elem, atree); }
 
         //Toggle on, previously was toggled off
         toggle_tab('atree-dropdown'); toggleButton('toggle-atree');
@@ -185,6 +193,17 @@ const atree_render = new (class extends ComputeNode {
     }
 })().link_to(atree_node);
 
+// This exists so i don't have to re-render the UI to push atree updates.
+const atree_state_node = new (class extends ComputeNode {
+    constructor() { super('builder-atree-state'); }
+
+    compute_func(input_map) {
+        if (input_map.size !== 1) { throw "AbilityTreeStateNode accepts exactly one input (atree-rendered)"; }
+        const [rendered_atree] = input_map.values();  // Extract values, pattern match it into size one list and bind to first element
+        return rendered_atree;
+    }
+})().link_to(atree_render, 'atree-render');
+
 /**
  * Create a reverse topological sort of the tree in the result list.
  *
@@ -266,7 +285,7 @@ const atree_merge = new (class extends ComputeNode {
         }
         return abils_merged;
     }
-})().link_to(atree_node, 'atree').link_to(atree_render, 'atree-state'); // TODO: THIS IS WRONG!!!!! Need one "collect" node...
+})().link_to(atree_node, 'atree').link_to(atree_state_node, 'atree-state'); // TODO: THIS IS WRONG!!!!! Need one "collect" node...
 
 /**
  * Validate ability tree.
@@ -284,9 +303,12 @@ const atree_validate = new (class extends ComputeNode {
         let errors = [];
         let reachable = new Map();
         atree_dfs_mark(atree_order[0], atree_state, reachable);
+        let abil_points_total = 0;
+        let archetype_count = new Map();
         for (const node of atree_order) {
             const abil = node.ability;
             if (!atree_state.get(abil.id).active) { continue; }
+            abil_points_total += abil.cost;
             if (!reachable.get(abil.id)) { errors.push(abil.display_name + ' is not reachable!'); }
 
             let failed_deps = [];
@@ -294,8 +316,8 @@ const atree_validate = new (class extends ComputeNode {
                 if (!atree_state.get(dep_id).active) { failed_deps.push(dep_id) }
             }
             if (failed_deps.length > 0) {
-                const dep_string = failed_deps.map(i => '"' + atree_state.get(i).ability.name + '"');
-                errors.push(abil.name + ' dependencies not satisfied: ' + dep_string.join(", "));
+                const dep_string = failed_deps.map(i => '"' + atree_state.get(i).ability.display_name + '"');
+                errors.push(abil.display_name + ' dependencies not satisfied: ' + dep_string.join(", "));
             }
 
             let blocking_ids = [];
@@ -303,17 +325,41 @@ const atree_validate = new (class extends ComputeNode {
                 if (atree_state.get(blocker_id).active) { blocking_ids.push(blocker_id); }
             }
             if (blocking_ids.length > 0) {
-                const blockers_string = blocking_ids.map(i => '"' + atree_state.get(i).ability.name + '"');
-                errors.push(abil.name + ' is blocked by: ' + blockers_string.join(", "));
+                const blockers_string = blocking_ids.map(i => '"' + atree_state.get(i).ability.display_name + '"');
+                errors.push(abil.display_name+' is blocked by: '+blockers_string.join(", "));
+            }
+
+            if ('archetype' in abil && abil.archetype !== "") {
+                let val = 1;
+                if (archetype_count.has(abil.archetype)) {
+                    val = archetype_count.get(abil.archetype) + 1;
+                }
+                archetype_count.set(abil.archetype, val);
+            }
+        }
+        // TODO: FIX THIS! ARCHETYPE REQ IS A PAIN IN THE ASS
+        // it doesn't follow topological order and theres some cases where "equip order" matters.
+        for (const node of atree_order) {
+            const abil = node.ability;
+            if (!atree_state.get(abil.id).active) { continue; }
+            if ('archetype_req' in abil && abil.archetype_req !== 0) {
+                const others = archetype_count.get(abil.archetype) - 1;
+                if (others < abil.archetype_req) {
+                    errors.push(abil.display_name+' fails archetype: '+abil.archetype+': '+others+' < '+abil.archetype_req)
+                }
             }
         }
 
-        return errors;
+        if (abil_points_total > 45) {
+            errors.push('too many ability points assigned! ('+abil_points_total+' > 45)');
+        }
+
+        return [abil_points_total, errors];
     }
-})().link_to(atree_node, 'atree').link_to(atree_render, 'atree-state');
+})().link_to(atree_node, 'atree').link_to(atree_state_node, 'atree-state');
 
 function atree_dfs_mark(start, atree_state, mark) {
-    if (mark.get(start)) { return; }
+    if (mark.get(start.ability.id)) { return; }
     mark.set(start.ability.id, true);
     for (const child of start.children) {
         if (atree_state.get(child.ability.id).active) {
@@ -331,9 +377,12 @@ const atree_render_active = new (class extends ComputeNode {
     compute_func(input_map) {
         const merged_abils = input_map.get('atree-merged');
         const atree_order = input_map.get('atree-order');
-        const errors = input_map.get('atree-errors');
+        const [abil_points_total, errors] = input_map.get('atree-errors');
 
         this.list_elem.innerHTML = ""; //reset all atree actives - should be done in a more general way later
+        // TODO: move to display?
+        document.getElementById("active_AP_cost").textContent = abil_points_total;
+
         if (errors.length > 0) {
             let errorbox = document.createElement('div');
             errorbox.classList.add("rounded-bottom", "dark-4", "border", "p-0", "mx-2", "my-4", "dark-shadow");
@@ -543,7 +592,6 @@ class AbilityTreeEnsureNodesNode extends ComputeNode {
     }
 }
 
-let abil_points_current;
 /** The main function for rendering an ability tree. 
  * 
  * @param {Element} UI_elem - the DOM element to draw the atree within.
@@ -552,13 +600,9 @@ let abil_points_current;
  */
 function render_AT(UI_elem, list_elem, tree) {
     console.log("constructing ability tree UI");
-    list_elem.innerHTML = ""; //reset all atree actives - should be done in a more general way later
-    UI_elem.innerHTML = ""; //reset the atree in the DOM
-
     // add in the "Active" title to atree
     let active_row = document.createElement("div");
     active_row.classList.add("row", "item-title", "mx-auto", "justify-content-center");
-    abil_points_current = 0;
     let active_word = document.createElement("div");
     active_word.classList.add("col-auto");
     active_word.textContent = "Active Abilities:";
@@ -706,29 +750,33 @@ function render_AT(UI_elem, list_elem, tree) {
 
         node_elem.classList.add("fake-button");
 
-        let active_tooltip = document.createElement('div');
-        active_tooltip.classList.add("rounded-bottom", "dark-4", "border", "p-0", "mx-2", "my-4", "dark-shadow");
-        active_tooltip.style.display = "none";
+        let node_tooltip = document.createElement('div');
+        node_tooltip.classList.add("rounded-bottom", "dark-4", "border", "p-0", "mx-2", "my-4", "dark-shadow");
+        node_tooltip.style.display = "none";
 
         // tooltip text formatting
 
-        let active_tooltip_title = document.createElement('b');
-        active_tooltip_title.classList.add("scaled-font");
-        active_tooltip_title.innerHTML = ability.display_name;
+        let node_tooltip_title = document.createElement('b');
+        node_tooltip_title.classList.add("scaled-font");
+        node_tooltip_title.innerHTML = ability.display_name;
+        node_tooltip.appendChild(node_tooltip_title);
 
-        let active_tooltip_desc = document.createElement('p');
-        active_tooltip_desc.classList.add("scaled-font-sm", "my-0", "mx-1", "text-wrap");
-        active_tooltip_desc.textContent = ability.desc;
+        if ('archetype' in ability && ability.archetype !== "") {
+            let node_tooltip_archetype = document.createElement('p');
+            node_tooltip_archetype.classList.add("scaled-font");
+            node_tooltip_archetype.innerHTML = "(Archetype: " + ability.archetype+")";
+            node_tooltip.appendChild(node_tooltip_archetype);
+        }
 
-        let active_tooltip_cost = document.createElement('p');
-        active_tooltip_cost.classList.add("scaled-font-sm", "my-0", "mx-1", "text-start");
-        active_tooltip_cost.textContent = "Cost: " + ability.cost + " AP";
+        let node_tooltip_desc = document.createElement('p');
+        node_tooltip_desc.classList.add("scaled-font-sm", "my-0", "mx-1", "text-wrap");
+        node_tooltip_desc.textContent = ability.desc;
+        node_tooltip.appendChild(node_tooltip_desc);
 
-        active_tooltip.appendChild(active_tooltip_title);
-        active_tooltip.appendChild(active_tooltip_desc);
-        active_tooltip.appendChild(active_tooltip_cost);
-
-        node_tooltip = active_tooltip;//.cloneNode(true);
+        let node_tooltip_cost = document.createElement('p');
+        node_tooltip_cost.classList.add("scaled-font-sm", "my-0", "mx-1", "text-start");
+        node_tooltip_cost.textContent = "Cost: " + ability.cost + " AP";
+        node_tooltip.appendChild(node_tooltip_cost);
 
         node_tooltip.style.position = "absolute";
         node_tooltip.style.zIndex = "100";
@@ -736,22 +784,13 @@ function render_AT(UI_elem, list_elem, tree) {
         node_elem.appendChild(node_tooltip);
         //list_elem.appendChild(active_tooltip);    NOTE: moved to `atree_render_active`
 
+        node_wrap.elem = node_elem;
+        node_wrap.all_connectors_ref = atree_connectors_map;
+
         node_elem.addEventListener('click', function(e) {
             if (e.target !== this && e.target!== this.children[0]) {return;}
-            if (node_wrap.active) {
-                this.classList.remove("atree-selected");
-                abil_points_current -= ability.cost;
-            } 
-            else {
-                this.classList.add("atree-selected");
-                abil_points_current += ability.cost;
-            };
-            document.getElementById("active_AP_cost").textContent = abil_points_current;
-            atree_toggle_state(atree_connectors_map, node_wrap);
-            atree_merge.mark_dirty();
-            atree_validate.mark_dirty();
-            atree_merge.update();
-            atree_validate.update();
+            atree_set_state(node_wrap, !node_wrap.active);
+            atree_state_node.mark_dirty().update();
         });
 
         // add tooltip
@@ -841,9 +880,16 @@ function atree_render_connection(atree_connectors_map) {
 };
 
 // toggle the state of a node.
-function atree_toggle_state(atree_connectors_map, node_wrapper) {
-    const new_state = !node_wrapper.active;
-    node_wrapper.active = new_state
+function atree_set_state(node_wrapper, new_state) {
+    if (new_state) {
+        node_wrapper.active = true;
+        node_wrapper.elem.classList.add("atree-selected");
+    } 
+    else {
+        node_wrapper.active = false;
+        node_wrapper.elem.classList.remove("atree-selected");
+    }
+    let atree_connectors_map = node_wrapper.all_connectors_ref;
     for (const parent of node_wrapper.parents) {
         if (parent.active) {
             atree_set_edge(atree_connectors_map, parent, node_wrapper, new_state);  // self->parent state only changes if parent is on
diff --git a/js/atree_constants.js b/js/atree_constants.js
index 46997df..57c23f6 100644
--- a/js/atree_constants.js
+++ b/js/atree_constants.js
@@ -45,8 +45,6 @@ const atrees = {
         {
             "display_name": "Escape",
             "desc": "Throw yourself backward to avoid danger. (Hold shift while escaping to cancel)",
-            "archetype": "", 
-            "archetype_req": 0, 
             "parents": ["Heart Shatter"], 
             "dependencies": [],
             "blockers": [],
@@ -141,8 +139,6 @@ const atrees = {
         {
             "display_name": "Fire Creep",
             "desc": "Arrow Bomb will leak a trail of fire for 6s, Damaging enemies that walk into it every 0.4s.",
-            "archetype": "", 
-            "archetype_req": 0, 
             "parents": ["Phantom Ray", "Fire Mastery", "Bryophyte Roots"], 
             "dependencies": [], 
             "blockers": [],
diff --git a/js/build_encode_decode.js b/js/build_encode_decode.js
index c7c42c4..bf7be38 100644
--- a/js/build_encode_decode.js
+++ b/js/build_encode_decode.js
@@ -20,6 +20,8 @@ function parsePowdering(powder_info) {
     return [powdering, powder_info];
 }
 
+let atree_data = null;
+
 /*
  * Populate fields based on url, and calculate build.
  */
@@ -62,7 +64,7 @@ function decodeBuild(url_tag) {
             }
             info[1] = info_str.slice(start_idx);
         }
-        else if (version_number <= 6) {
+        else if (version_number <= 7) {
             let info_str = info[1];
             let start_idx = 0;
             for (let i = 0; i < 9; ++i ) {
@@ -101,7 +103,7 @@ function decodeBuild(url_tag) {
             let powder_info = info[1].slice(10);
             let res = parsePowdering(powder_info);
             powdering = res[0];
-        } else if (version_number <= 6){
+        } else if (version_number <= 7){
             level = Base64.toInt(info[1].slice(10,12));
             setValue("level-choice",level);
             save_skp = true;
@@ -117,7 +119,7 @@ function decodeBuild(url_tag) {
             info[1] = res[1];
         }
         // Tomes.
-        if (version == 6) {
+        if (version >= 6) {
             //tome values do not appear in anything before v6.
             for (let i in tomes) {
                 let tome_str = info[1].charAt(i);
@@ -128,6 +130,14 @@ function decodeBuild(url_tag) {
             info[1] = info[1].slice(7);
         }
 
+        if (version >= 7) {
+            // ugly af. only works since its the last thing. will be fixed with binary decode
+            atree_data = new BitVector(info[1]);
+        }
+        else {
+            atree_data = null;
+        }
+
         for (let i in powder_inputs) {
             setValue(powder_inputs[i], powdering[i]);
         }
@@ -139,12 +149,13 @@ function decodeBuild(url_tag) {
 
 /*  Stores the entire build in a string using B64 encoding and adds it to the URL.
 */
-function encodeBuild(build, powders, skillpoints) {
+function encodeBuild(build, powders, skillpoints, atree, atree_state) {
 
     if (build) {
         let build_string;
         
         //V6 encoding - Tomes
+        //V7 encoding - ATree
         build_version = 5;
         build_string = "";
         tome_string = "";
@@ -189,6 +200,12 @@ function encodeBuild(build, powders, skillpoints) {
         }
         build_string += tome_string;
 
+        if (atree_state.get(atree[0].ability.id).active) {
+            build_version = Math.max(build_version, 7);
+            const bitvec = encode_atree(atree, atree_state);
+            build_string += bitvec.toB64();
+        }
+
         return build_version.toString() + "_" + build_string;
     }
 }
@@ -216,3 +233,58 @@ function shareBuild(build) {
     }
 }
 
+/**
+ * Ability tree encode and decode functions
+ *
+ * Based on a traversal, basically only uses bits to represent the nodes that are on (and "dark" outgoing edges).
+ * credit: SockMower
+ */
+
+/**
+ * Return: BitVector
+ */
+function encode_atree(atree, atree_state) {
+    let ret_vec = new BitVector(0, 0);
+
+    function traverse(head, atree_state, visited, ret) {
+        for (const child of head.children) {
+            if (visited.has(child.ability.id)) { continue; }
+            visited.set(child.ability.id, true);
+            if (atree_state.get(child.ability.id).active) {
+                ret.append(1, 1);
+                traverse(child, atree_state, visited, ret);
+            }
+            else {
+                ret.append(0, 1);
+            }
+        }
+    }
+
+    traverse(atree[0], atree_state, new Map(), ret_vec);
+    return ret_vec;
+}
+
+/**
+ * Return: List of active nodes
+ */
+function decode_atree(atree, bits) {
+    let i = 0;
+    let ret = [];
+    ret.push(atree[0]);
+    function traverse(head, visited, ret) {
+        for (const child of head.children) {
+            if (visited.has(child.ability.id)) { continue; }
+            visited.set(child.ability.id, true);
+            if (bits.read_bit(i)) {
+                i += 1;
+                ret.push(child);
+                traverse(child, visited, ret);
+            }
+            else {
+                i += 1;
+            }
+        }
+    }
+    traverse(atree[0], new Map(), ret);
+    return ret;
+}
diff --git a/js/builder_graph.js b/js/builder_graph.js
index e541662..4a0384b 100644
--- a/js/builder_graph.js
+++ b/js/builder_graph.js
@@ -344,6 +344,8 @@ class BuildEncodeNode extends ComputeNode {
 
     compute_func(input_map) {
         const build = input_map.get('build');
+        const atree = input_map.get('atree');
+        const atree_state = input_map.get('atree-state');
         let powders = [
             input_map.get('helmet-powder'),
             input_map.get('chestplate-powder'),
@@ -361,7 +363,7 @@ class BuildEncodeNode extends ComputeNode {
         // TODO: grr global state for copy button..
         player_build = build;
         build_powders = powders;
-        return encodeBuild(build, powders, skillpoints);
+        return encodeBuild(build, powders, skillpoints, atree, atree_state);
     }
 }
 
@@ -1077,6 +1079,8 @@ function builder_graph_init() {
     atree_graph_creator = new AbilityTreeEnsureNodesNode(build_node, stat_agg_node)
                                     .link_to(atree_collect_spells, 'spells');
 
+    build_encode_node.link_to(atree_node, 'atree').link_to(atree_state_node, 'atree-state');
+
     // ---------------------------------------------------------------
     //  Trigger the update cascade for build!
     // ---------------------------------------------------------------
@@ -1085,6 +1089,18 @@ function builder_graph_init() {
     }
     level_input.update();
 
+    // kinda janky, manually set atree and update. Some wasted compute here
+    if (atree_data !== null && atree_node.value !== null) { // janky check if atree is valid
+        const atree_state = atree_state_node.value;
+        if (atree_data.length > 0) {
+            const active_nodes = decode_atree(atree_node.value, atree_data);
+            for (const node of active_nodes) {
+                atree_set_state(atree_state.get(node.ability.id), true);
+            }
+            atree_state_node.mark_dirty().update();
+        }
+    }
+
     // Powder specials.
     let powder_special_calc = new PowderSpecialCalcNode().link_to(powder_special_input, 'powder-specials');
     new PowderSpecialDisplayNode().link_to(powder_special_input, 'powder-specials')
diff --git a/js/display.js b/js/display.js
index 0e5ba09..19c252a 100644
--- a/js/display.js
+++ b/js/display.js
@@ -1603,7 +1603,7 @@ function displaySpellDamage(parent_elem, overallparent_elem, stats, spell, spell
         
 
         let third = document.createElement("span");
-        third.textContent = ") [Base: " + getBaseSpellCost(stats, spellIdx, spell.cost) + " ]";
+        third.textContent = ")";// [Base: " + getBaseSpellCost(stats, spellIdx, spell.cost) + " ]";
         title_elem.appendChild(third);
         let third_summary = document.createElement("span");
         third_summary.textContent = ")";
diff --git a/js/utils.js b/js/utils.js
index ece2acf..052a784 100644
--- a/js/utils.js
+++ b/js/utils.js
@@ -193,9 +193,9 @@ Base64 = (function () {
      */
     read_bit(idx) {
         if (idx < 0 || idx >= this.length) {
-            throw new RangeError("Cannot read bit outside the range of the BitVector.");
+            throw new RangeError("Cannot read bit outside the range of the BitVector. ("+idx+" > "+this.length+")");
         }
-        return ((this.bits[Math.floor(idx / 32)] & (1 << (idx % 32))) == 0 ? 0 : 1);
+        return ((this.bits[Math.floor(idx / 32)] & (1 << idx)) == 0 ? 0 : 1);
     }
 
     /** Returns an integer value (if possible) made from the range of bits [start, end). Undefined behavior if the range to read is too big.