Revert original hashing. Prepare 3 versions of hashing in the 'lib'.

circuits/nova_snark/blur_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 166
-non-linear constraints: 253110
+template instances: 88
+non-linear constraints: 345126
 linear constraints: 0
 public inputs: 4
 private inputs: 512 (none belong to witness)
 public outputs: 4
-wires: 245433
-labels: 619532
+wires: 337449
+labels: 866504
 Written successfully: ./blur_step.r1cs
 Written successfully: ./blur_step_js/blur_step.wasm
 Everything went okay

circuits/nova_snark/brightness_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 165
-non-linear constraints: 307272
+template instances: 87
+non-linear constraints: 353280
 linear constraints: 1
 public inputs: 3
 private inputs: 256
 public outputs: 3
-wires: 291917
-labels: 606134
+wires: 337925
+labels: 729620
 Written successfully: ./brightness_step.r1cs
 Written successfully: ./brightness_step_js/brightness_step.wasm
 Everything went okay

circuits/nova_snark/contrast_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 165
-non-linear constraints: 307272
+template instances: 87
+non-linear constraints: 353280
 linear constraints: 1
 public inputs: 3
 private inputs: 256
 public outputs: 3
-wires: 291917
-labels: 606134
+wires: 337925
+labels: 729620
 Written successfully: ./contrast_step.r1cs
 Written successfully: ./contrast_step_js/contrast_step.wasm
 Everything went okay

circuits/nova_snark/crop_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 176
-non-linear constraints: 673718
+template instances: 97
+non-linear constraints: 708224
 linear constraints: 1
 public inputs: 3
 private inputs: 128
 public outputs: 3
-wires: 673079
-labels: 2596364
+wires: 707585
+labels: 2688977
 Written successfully: ./crop_step.r1cs
 Written successfully: ./crop_step_js/crop_step.wasm
 Everything went okay

circuits/nova_snark/grayscale_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 164
-non-linear constraints: 122952
+template instances: 86
+non-linear constraints: 168960
 linear constraints: 0
 public inputs: 2
 private inputs: 256
 public outputs: 2
-wires: 120395
-labels: 260401
+wires: 166403
+labels: 383887
 Written successfully: ./grayscale_step.r1cs
 Written successfully: ./grayscale_step_js/grayscale_step.wasm
 Everything went okay

circuits/nova_snark/hash_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 154
-non-linear constraints: 7716
+template instances: 76
+non-linear constraints: 30720
 linear constraints: 0
 public inputs: 1
 private inputs: 128
 public outputs: 1
-wires: 7846
-labels: 37463
+wires: 30850
+labels: 99206
 Written successfully: ./hash_step.r1cs
 Written successfully: ./hash_step_js/hash_step.wasm
 Everything went okay

circuits/nova_snark/resize_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 163
-non-linear constraints: 245904
+template instances: 84
+non-linear constraints: 337920
 linear constraints: 0
 public inputs: 2
 private inputs: 512 (none belong to witness)
 public outputs: 2
-wires: 238227
-labels: 596313
+wires: 330243
+labels: 843282
 Written successfully: ./resize_step.r1cs
 Written successfully: ./resize_step_js/resize_step.wasm
 Everything went okay

circuits/nova_snark/sharpness_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 171
-non-linear constraints: 329910
+template instances: 93
+non-linear constraints: 421926
 linear constraints: 0
 public inputs: 4
 private inputs: 512 (128 belong to witness)
 public outputs: 4
-wires: 314553
-labels: 803852
+wires: 406569
+labels: 1050824
 Written successfully: ./sharpness_step.r1cs
 Written successfully: ./sharpness_step_js/sharpness_step.wasm
 Everything went okay

circuits/sonobe/blur_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 167
-non-linear constraints: 253110
+template instances: 89
+non-linear constraints: 345126
 linear constraints: 0
 public inputs: 4
 private inputs: 512 (none belong to witness)
 public outputs: 4
-wires: 245433
-labels: 621068
+wires: 337449
+labels: 868040
 Written successfully: ./blur_step.r1cs
 Written successfully: ./blur_step_js/blur_step.wasm
 Everything went okay

circuits/sonobe/brightness_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 166
-non-linear constraints: 307272
+template instances: 88
+non-linear constraints: 353280
 linear constraints: 1
 public inputs: 3
 private inputs: 256
 public outputs: 3
-wires: 291917
-labels: 606902
+wires: 337925
+labels: 730388
 Written successfully: ./brightness_step.r1cs
 Written successfully: ./brightness_step_js/brightness_step.wasm
 Everything went okay

circuits/sonobe/contrast_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 166
-non-linear constraints: 307272
+template instances: 88
+non-linear constraints: 353280
 linear constraints: 1
 public inputs: 3
 private inputs: 256
 public outputs: 3
-wires: 291917
-labels: 606902
+wires: 337925
+labels: 730388
 Written successfully: ./contrast_step.r1cs
 Written successfully: ./contrast_step_js/contrast_step.wasm
 Everything went okay

circuits/sonobe/crop_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 176
-non-linear constraints: 673718
+template instances: 97
+non-linear constraints: 708224
 linear constraints: 1
 public inputs: 3
 private inputs: 128
 public outputs: 3
-wires: 673079
-labels: 2596364
+wires: 707585
+labels: 2688977
 Written successfully: ./crop_step.r1cs
 Written successfully: ./crop_step_js/crop_step.wasm
 Everything went okay

circuits/sonobe/grayscale_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 165
-non-linear constraints: 122952
+template instances: 87
+non-linear constraints: 168960
 linear constraints: 0
 public inputs: 2
 private inputs: 256
 public outputs: 2
-wires: 120395
-labels: 261169
+wires: 166403
+labels: 384655
 Written successfully: ./grayscale_step.r1cs
 Written successfully: ./grayscale_step_js/grayscale_step.wasm
 Everything went okay

circuits/sonobe/hash_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 154
-non-linear constraints: 7716
+template instances: 76
+non-linear constraints: 30720
 linear constraints: 0
 public inputs: 1
 private inputs: 128
 public outputs: 1
-wires: 7846
-labels: 37463
+wires: 30850
+labels: 99206
 Written successfully: ./hash_step.r1cs
 Written successfully: ./hash_step_js/hash_step.wasm
 Everything went okay

circuits/sonobe/resize_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 164
-non-linear constraints: 245904
+template instances: 85
+non-linear constraints: 337920
 linear constraints: 0
 public inputs: 2
 private inputs: 512 (none belong to witness)
 public outputs: 2
-wires: 238227
-labels: 597849
+wires: 330243
+labels: 844818
 Written successfully: ./resize_step.r1cs
 Written successfully: ./resize_step_js/resize_step.wasm
 Everything went okay

circuits/sonobe/sharpness_step.compile_log

@@ -1,11 +1,11 @@
-template instances: 172
-non-linear constraints: 329910
+template instances: 94
+non-linear constraints: 421926
 linear constraints: 0
 public inputs: 4
 private inputs: 512 (128 belong to witness)
 public outputs: 4
-wires: 314553
-labels: 805388
+wires: 406569
+labels: 1052360
 Written successfully: ./sharpness_step.r1cs
 Written successfully: ./sharpness_step_js/sharpness_step.wasm
 Everything went okay

circuits/src/utils/hashers.circom

@@ -1,3 +1,5 @@
+// Mini-library exposing various hasher templates. They impement Poseidon hashing for different input types.
+
 pragma circom 2.0.0;
 include "../../node_modules/circomlib/circuits/poseidon.circom";
 
@@ -12,50 +14,109 @@ template PairHasher() {
     hash <== hasher.out;
 }
 
-// Compute Poseidon hash of an array of values using a chunk size of 8.
-template ArrayHasher(length) {
-    signal input array[length];
+// Compute Poseidon hash of an array of values.
+template ArrayHasher(LENGTH) {
+    signal input  array[LENGTH];
+    signal output hash;
+    hash <== _LinearFoldHasher(LENGTH)(array);
+//    hash <== _WindowFoldHasher(LENGTH, 8)(array);
+//    hash <== _MerkleHasher(LENGTH, 8)(array);
+}
+
+// Compute Poseidon hash of an array by folding the it *one element at a time*.
+template _LinearFoldHasher(LENGTH) {
+    signal input  array[LENGTH];
+    signal output hash;
+
+    signal intermediate_hash[LENGTH-1];
+
+    intermediate_hash[0] <== PairHasher()(array[0], array[1]);
+    for(var i = 1; i < LENGTH-1; i++) {
+        intermediate_hash[i] <== PairHasher()(intermediate_hash[i-1], array[i+1]);
+    }
+
+    hash <== intermediate_hash[LENGTH-2];
+}
+
+// Compute Poseidon hash of an array by folding it with a *window of size `WINDOW_SIZE`*.
+template _WindowFoldHasher(LENGTH, WINDOW_SIZE) {
+    signal input  array[LENGTH];
     signal output hash;
 
-    if (length == 1) {
+    var numRounds = (LENGTH + WINDOW_SIZE - 1) \ WINDOW_SIZE;
+
+    signal intermediateHashes[numRounds];
+
+    // Process the first window
+    var firstWindowSize = (LENGTH < WINDOW_SIZE) ? LENGTH : WINDOW_SIZE;
+    component firstHasher = Poseidon(firstWindowSize);
+    for (var i = 0; i < firstWindowSize; i++) {
+        firstHasher.inputs[i] <== array[i];
+    }
+    intermediateHashes[0] <== firstHasher.out;
+
+    // Process subsequent windows
+    var processed = firstWindowSize;
+    component hashers[numRounds - 1];
+    for (var foldRound = 0; foldRound < numRounds - 1; foldRound++) {
+        var remaining = LENGTH - processed;
+        var currentWindowSize = (remaining < WINDOW_SIZE - 1) ? remaining : WINDOW_SIZE - 1;
+
+        hashers[foldRound] = Poseidon(currentWindowSize + 1);
+        hashers[foldRound].inputs[0] <== intermediateHashes[foldRound];
+        for (var i = 0; i < currentWindowSize; i++) {
+            hashers[foldRound].inputs[i + 1] <== array[processed + i];
+        }
+        intermediateHashes[foldRound + 1] <== hashers[foldRound].out;
+        processed += currentWindowSize;
+    }
+
+    hash <== intermediateHashes[numRounds - 1];
+}
+
+// Compute Poseidon hash of an array by computing `ARITY`-ary Merkle tree of hashes over it.
+template _MerkleHasher(LENGTH, ARITY) {
+    signal input  array[LENGTH];
+    signal output hash;
+
+    var fullChunks = LENGTH \ ARITY;
+    var remainingElements = LENGTH % ARITY;
+    var chunks = fullChunks + (remainingElements > 0 ? 1 : 0);
+
+    if (LENGTH == 1) {
         hash <== array[0];
     } else {
-        var chunkSize = 8;
+        // Hashes of disjoint chunks of length `ARITY` (except possibly the last one, which might be shorter).
+        signal chunkOutputs[chunks];
 
-        var fullChunks = length \ chunkSize;
+        // Handle full chunks.
         component fullHashers[fullChunks];
         for (var i = 0; i < fullChunks; i++) {
-            fullHashers[i] = Poseidon(chunkSize);
-            for (var j = 0; j < chunkSize; j++) {
-                fullHashers[i].inputs[j] <== array[i * chunkSize + j];
+            fullHashers[i] = Poseidon(ARITY);
+            for (var j = 0; j < ARITY; j++) {
+                fullHashers[i].inputs[j] <== array[i * ARITY + j];
             }
+            chunkOutputs[i] <== fullHashers[i].out;
         }
 
-        if (length % chunkSize == 0) {
-            component nextRound = ArrayHasher(fullChunks);
-            for (var i = 0; i < fullChunks; i++) {
-                nextRound.array[i] <== fullHashers[i].out;
+        // Handle remaining elements (if any).
+        if (remainingElements > 0) {
+            component partialHasher = Poseidon(remainingElements);
+            for (var i = 0; i < remainingElements; i++) {
+                partialHasher.inputs[i] <== array[fullChunks * ARITY + i];
             }
-            hash <== nextRound.hash;
-        } else {
-            component partialHasher = Poseidon(length % chunkSize);
-            for (var i = 0; i < length % chunkSize; i++) {
-                partialHasher.inputs[i] <== array[fullChunks * chunkSize + i];
-            }
-            component nextRound = ArrayHasher(fullChunks + 1);
-            for (var i = 0; i < fullChunks; i++) {
-                nextRound.array[i] <== fullHashers[i].out;
-            }
-            nextRound.array[fullChunks] <== partialHasher.out;
-            hash <== nextRound.hash;
+            chunkOutputs[fullChunks] <== partialHasher.out;
         }
+
+        // Recursively hash the chunk hashes.
+        hash <== _MerkleHasher(chunks, ARITY)(chunkOutputs);
     }
 }
 
 // Compute Poseidon hash of a single value and an array of values.
-template HeadTailHasher(tailLength) {
-    signal input  head, tail[tailLength];
+template HeadTailHasher(TAIL_LENGTH) {
+    signal input  head, tail[TAIL_LENGTH];
     signal output hash;
 
-    hash <== PairHasher()(head, ArrayHasher(tailLength)(tail));
+    hash <== PairHasher()(head, ArrayHasher(TAIL_LENGTH)(tail));
 }