// doc.js — the canvas document plus its undo stack.
//
// Every mutation passes through begin()/commit(). A gesture (a drag, a resize,
// a nudge-repeat) spans hundreds of pointer events but has to collapse into a
// single undo entry, so the snapshot is taken once at begin() and pushed once
// at commit() — and only when the document actually changed, so a click that
// moves nothing leaves no trace.
//
// Undo is snapshot-based rather than a stack of inverse operations: the
// document is a small pure-JSON object, cloning it costs microseconds, and a
// snapshot cannot be subtly wrong the way a hand-written inverse can. The
// stack is capped so a long session cannot grow without bound.
window.Canvas = window.Canvas || {};

Canvas.Doc = (function () {
    var MAX_UNDO = 100;

    var doc = { nodes: [], edges: [] };
    var undoStack = [];
    var redoStack = [];
    var pending = null;
    var dirty = false;
    var version = '';
    var listeners = [];

    function clone(v) { return JSON.parse(JSON.stringify(v)); }

    function emit() {
        for (var i = 0; i < listeners.length; i++) listeners[i]();
    }

    // newId mints a 16-hex-character node/edge id, the shape the format uses in
    // the wild. Collisions are checked against the live document anyway.
    function newId() {
        var bytes = new Uint8Array(8);
        window.crypto.getRandomValues(bytes);
        var out = '';
        for (var i = 0; i < bytes.length; i++) {
            out += (bytes[i] + 0x100).toString(16).slice(1);
        }
        if (find(doc.nodes, out) || find(doc.edges, out)) return newId();
        return out;
    }

    function find(list, id) {
        for (var i = 0; i < list.length; i++) {
            if (list[i].id === id) return list[i];
        }
        return null;
    }

    return {
        init: function (d, hash) {
            doc = (d && typeof d === 'object') ? d : {};
            if (!Array.isArray(doc.nodes)) doc.nodes = [];
            if (!Array.isArray(doc.edges)) doc.edges = [];
            undoStack = [];
            redoStack = [];
            pending = null;
            dirty = false;
            version = hash || '';
        },

        get: function () { return doc; },
        nodes: function () { return doc.nodes; },
        edges: function () { return doc.edges; },
        node: function (id) { return find(doc.nodes, id); },
        edge: function (id) { return find(doc.edges, id); },
        newId: newId,

        // edgesFor returns every edge touching any of the given node ids.
        edgesFor: function (nodeIds) {
            var want = {};
            nodeIds.forEach(function (id) { want[id] = true; });
            return doc.edges.filter(function (e) {
                return want[e.fromNode] || want[e.toNode];
            });
        },

        // --- gesture bracketing ---
        //
        // Nested begin() calls join the outer gesture rather than starting a
        // new one, so a composite action (drag a group, which drags its
        // members) still produces exactly one undo entry.
        begin: function (label) {
            if (pending) { pending.depth++; return; }
            pending = { label: label || '', before: clone(doc), depth: 0 };
        },

        commit: function () {
            if (!pending) return;
            if (pending.depth > 0) { pending.depth--; return; }
            var before = pending.before;
            var label = pending.label;
            pending = null;
            // A gesture that changed nothing (a click that selected without
            // moving) must not push an undo entry the user would have to press
            // Ctrl+Z through.
            if (JSON.stringify(before) === JSON.stringify(doc)) return;
            undoStack.push({ label: label, doc: before });
            if (undoStack.length > MAX_UNDO) undoStack.shift();
            redoStack.length = 0;
            dirty = true;
            emit();
        },

        // abort rolls the document back to the gesture's starting state, for
        // Escape mid-drag.
        abort: function () {
            if (!pending) return;
            if (pending.depth > 0) { pending.depth--; return; }
            doc = pending.before;
            pending = null;
            emit();
        },

        inGesture: function () { return pending !== null; },

        undo: function () {
            if (!undoStack.length) return false;
            var entry = undoStack.pop();
            redoStack.push({ label: entry.label, doc: clone(doc) });
            doc = entry.doc;
            dirty = true;
            emit();
            return true;
        },

        redo: function () {
            if (!redoStack.length) return false;
            var entry = redoStack.pop();
            undoStack.push({ label: entry.label, doc: clone(doc) });
            doc = entry.doc;
            dirty = true;
            emit();
            return true;
        },

        canUndo: function () { return undoStack.length > 0; },
        canRedo: function () { return redoStack.length > 0; },

        // --- structural helpers ---

        addNode: function (node) {
            doc.nodes.push(node);
            return node;
        },

        addEdge: function (edge) {
            doc.edges.push(edge);
            return edge;
        },

        // remove drops the named nodes and edges. Every edge touching a removed
        // node goes too — a dangling endpoint would fail validation on save, so
        // it can never be allowed to exist even transiently.
        remove: function (nodeIds, edgeIds) {
            var deadNodes = {};
            (nodeIds || []).forEach(function (id) { deadNodes[id] = true; });
            var deadEdges = {};
            (edgeIds || []).forEach(function (id) { deadEdges[id] = true; });

            doc.nodes = doc.nodes.filter(function (n) { return !deadNodes[n.id]; });
            doc.edges = doc.edges.filter(function (e) {
                return !deadEdges[e.id] && !deadNodes[e.fromNode] && !deadNodes[e.toNode];
            });
        },

        // --- save state ---

        isDirty: function () { return dirty; },
        markSaved: function (hash) {
            dirty = false;
            if (hash) version = hash;
            emit();
        },
        version: function () { return version; },

        onChange: function (fn) { listeners.push(fn); },
        notify: emit
    };
})();