Add MA decoder

src/ImageSharp/Formats/Jxl/Processing/Modular/Encoding/JxlMaDecoder.cs

@@ -0,0 +1,196 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
+using SixLabors.ImageSharp.Formats.Jxl.Processing.Modular.Encoding.ContextPrediction;
+using SixLabors.ImageSharp.Formats.Jxl.Processing.Primitives;
+using Tree = System.Collections.Generic.List<SixLabors.ImageSharp.Formats.Jxl.Processing.Modular.Encoding.JxlPropertyDecisionNode>;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Modular.Encoding;
+
+internal static class JxlMaDecoder
+{
+    private enum NextAction
+    {
+        CheckAndGoLeft,
+        GoRight,
+        Pop
+    }
+
+    public static void ValidateTree(Tree tree)
+    {
+        const int heightLimit = 2048;
+
+        if (tree.Count == 0)
+        {
+            return;
+        }
+
+        int numProperties = tree.Max(x => x.Property + 1);
+        Span<(int First, int Second)> propertyRanges = stackalloc (int First, int Second)[numProperties];
+
+        for (int i = 0; i < numProperties; i++)
+        {
+            propertyRanges[i].First = int.MinValue;
+            propertyRanges[i].Second = int.MaxValue;
+        }
+
+        Stack<WorkItem> stack = [];
+        stack.Push(new WorkItem(0, 0, 0, NextAction.CheckAndGoLeft));
+
+        while (stack.Count > 0)
+        {
+            if (stack.Count > heightLimit)
+            {
+                throw new InvalidOperationException("Tree too tall");
+            }
+
+            WorkItem item = stack.Peek();
+            JxlPropertyDecisionNode node = tree[item.NodeIndex];
+
+            switch (item.Action)
+            {
+                case NextAction.CheckAndGoLeft:
+                {
+                    int p = node.Property;
+                    if (p == -1)
+                    {
+                        _ = stack.Pop();
+                        continue;
+                    }
+
+                    int v = node.SplitValue;
+                    int l = propertyRanges[p].First;
+                    int u = propertyRanges[p].Second;
+
+                    if (l > v || u <= v)
+                    {
+                        throw new InvalidOperationException("Invalid tree");
+                    }
+
+                    item.OriginalL = l;
+                    item.OriginalU = u;
+                    item.Action = NextAction.GoRight;
+                    propertyRanges[node.Property].First = node.SplitValue + 1;
+
+                    stack.Push(new WorkItem(node.LeftChild, 0, 0, NextAction.CheckAndGoLeft));
+                    continue;
+                }
+
+                case NextAction.GoRight:
+                    item.Action = NextAction.Pop;
+                    propertyRanges[node.Property].First = item.OriginalL;
+                    propertyRanges[node.Property].Second = node.SplitValue;
+                    stack.Push(new WorkItem(node.LeftChild, 0, 0, NextAction.CheckAndGoLeft));
+                    continue;
+
+                case NextAction.Pop:
+                    propertyRanges[node.Property].Second = item.OriginalU;
+                    _ = stack.Pop();
+                    continue;
+            }
+        }
+    }
+
+    public static void DecodeTree(JxlBitReader br, JxlAnsSymbolReader reader, List<byte> contextMap, Tree tree, int treeSizeLimit)
+    {
+        int leafId = 0;
+        int toDecode = 1;
+        tree.Clear();
+
+        while (toDecode > 0)
+        {
+            if (tree.Count > treeSizeLimit)
+            {
+                throw new InvalidOperationException("Tree is too large");
+            }
+
+            toDecode--;
+            int prop1 = reader.ReadHybridUint(JxlMaTreeContext.Property, br, contextMap);
+            if (prop1 > 256)
+            {
+                throw new InvalidOperationException("Invalid tree property value");
+            }
+
+            int property = prop1 - 1;
+            if (property == -1)
+            {
+                int predictor = reader.ReadHybridUint(JxlMaTreeContext.Predictor, br, contextMap);
+
+                if (predictor >= JxlPredictorFacts.ModularPredictors)
+                {
+                    throw new InvalidOperationException("Invalid predictor");
+                }
+
+                int predictor_offset = JxlPackSigned.UnpackSigned(reader.ReadHybridUint(JxlMaTreeContext.Offset, br, contextMap));
+                int mul_log = reader.ReadHybridUint(JxlMaTreeContext.MultiplierLog, br, contextMap);
+
+                if (mul_log >= 31)
+                {
+                    throw new InvalidOperationException("Invalid multiplier logarithm");
+                }
+
+                int mul_bits = reader.ReadHybridUint(JxlMaTreeContext.MultiplierBits, br, contextMap);
+
+                if (mul_bits >= (1 << (31 - mul_log)) - 1)
+                {
+                    throw new InvalidOperationException("Invalid multiplier");
+                }
+
+                int multiplier = (mul_bits + 1) << mul_log;
+                JxlPredictor p = (JxlPredictor)predictor;
+
+                tree.Add(new JxlPropertyDecisionNode(-1, 0, leafId, 0, p, predictor_offset, multiplier));
+                leafId++;
+                continue;
+            }
+
+            int splitval = JxlPackSigned.UnpackSigned(reader.ReadHybridUint(JxlMaTreeContext.SplitValue, br, contextMap));
+            tree.Add(
+                new JxlPropertyDecisionNode(
+                    property,
+                    splitval,
+                    tree.Count + toDecode + 1,
+                    tree.Count + toDecode + 2,
+                    JxlPredictor.Zero,
+                    0,
+                    1));
+
+            toDecode += 2;
+        }
+
+        ValidateTree(tree);
+    }
+
+    public static void DecodeTree(Configuration configuration, JxlBitReader reader, Tree tree, int treeSizeLimit)
+    {
+        List<byte> treeContextMap = [];
+        JxlAnsCode code = new();
+
+        JxlAnsReader.DecodeHistograms(configuration, reader, JxlMaConstants.NumTreeContexts, code, treeContextMap);
+
+        if (code.DegenerateSymbols[treeContextMap[(int)JxlMaTreeContext.Property]] > 0)
+        {
+            throw new InvalidOperationException("Infinite tree");
+        }
+
+        JxlAnsSymbolReader symbolReader = JxlAnsSymbolReader.Create(code, reader);
+        DecodeTree(reader, symbolReader, treeContextMap, tree, Math.Min(treeSizeLimit, JxlMaConstants.MaxTreeSize));
+
+        if (!symbolReader.CheckAnsFinalState())
+        {
+            throw new InvalidOperationException("ANS decode final state failed");
+        }
+    }
+
+    private struct WorkItem(int nodeIndex, int origL, int origU, NextAction action)
+    {
+        public int NodeIndex { get; set; } = nodeIndex;
+
+        public int OriginalL { get; set; } = origL;
+
+        public int OriginalU { get; set; } = origU;
+
+        public NextAction Action { get; set; } = action;
+    }
+}