feat(patterns): pattern-based compression

Author: erights

packages/patterns/index.js

@@ -73,6 +73,7 @@ export {
   getInterfaceGuardPayload,
   getInterfaceMethodKeys,
 } from './src/patterns/getGuardPayloads.js';
+export { mustCompress, mustDecompress } from './src/patterns/compress.js';
 
 // eslint-disable-next-line import/export
 export * from './src/types.js';

packages/patterns/src/patterns/compress.js

@@ -0,0 +1,292 @@
+// @ts-nocheck So many errors that the suppressions hamper readability.
+// TODO fix and then turn at-ts-check back on
+import {
+  assertChecker,
+  makeTagged,
+  passStyleOf,
+  recordNames,
+  recordValues,
+} from '@endo/marshal';
+import {
+  kindOf,
+  assertPattern,
+  maybeMatchHelper,
+  matches,
+  checkMatches,
+  mustMatch,
+} from './patternMatchers.js';
+import { isKey } from '../keys/checkKey.js';
+import { keyEQ } from '../keys/compareKeys.js';
+
+/** @import {Compress, Decompress, MustCompress, MustDecompress} from '../types.js' */
+
+const { fromEntries } = Object;
+const { Fail, quote: q } = assert;
+
+const isNonCompressingMatcher = pattern => {
+  const patternKind = kindOf(pattern);
+  if (patternKind === undefined) {
+    return false;
+  }
+  const matchHelper = maybeMatchHelper(patternKind);
+  return matchHelper && matchHelper.compress === undefined;
+};
+
+/**
+ * When, for example, all the specimens in a given store match a
+ * specific pattern, then each of those specimens must contain the same
+ * literal superstructure as their one shared pattern. Therefore, storing
+ * that literal superstructure would be redumdant. If `specimen` does
+ * match `pattern`, then `compress(specimen, pattern)` will return a bindings
+ * array which is hopefully more compact than `specimen` as a whole, but
+ * carries all the information from specimen that cannot be derived just
+ * from knowledge that it matches this `pattern`.
+ *
+ * @type {Compress}
+ */
+const compress = (specimen, pattern) => {
+  if (isNonCompressingMatcher(pattern)) {
+    if (matches(specimen, pattern)) {
+      return harden({ compressed: specimen });
+    }
+    return undefined;
+  }
+
+  // Not yet frozen! Used to accumulate bindings
+  const bindings = [];
+  const emitBinding = binding => {
+    bindings.push(binding);
+  };
+  harden(emitBinding);
+
+  /**
+   * @param {Passable} innerSpecimen
+   * @param {Pattern} innerPattern
+   * @returns {boolean}
+   */
+  const compressRecur = (innerSpecimen, innerPattern) => {
+    assertPattern(innerPattern);
+    if (isKey(innerPattern)) {
+      return keyEQ(innerSpecimen, innerPattern);
+    }
+    const patternKind = kindOf(innerPattern);
+    const specimenKind = kindOf(innerSpecimen);
+    switch (patternKind) {
+      case undefined: {
+        return false;
+      }
+      case 'copyArray': {
+        if (
+          specimenKind !== 'copyArray' ||
+          innerSpecimen.length !== innerPattern.length
+        ) {
+          return false;
+        }
+        return innerPattern.every((p, i) => compressRecur(innerSpecimen[i], p));
+      }
+      case 'copyRecord': {
+        if (specimenKind !== 'copyRecord') {
+          return false;
+        }
+        const specimenNames = recordNames(innerSpecimen);
+        const pattNames = recordNames(innerPattern);
+
+        if (specimenNames.length !== pattNames.length) {
+          return false;
+        }
+        const specimenValues = recordValues(innerSpecimen, specimenNames);
+        const pattValues = recordValues(innerPattern, pattNames);
+
+        return pattNames.every(
+          (name, i) =>
+            specimenNames[i] === name &&
+            compressRecur(specimenValues[i], pattValues[i]),
+        );
+      }
+      case 'copyMap': {
+        if (specimenKind !== 'copyMap') {
+          return false;
+        }
+        const {
+          payload: { keys: pattKeys, values: valuePatts },
+        } = innerPattern;
+        const {
+          payload: { keys: specimenKeys, values: specimenValues },
+        } = innerSpecimen;
+        // TODO BUG: this assumes that the keys appear in the
+        // same order, so we can compare values in that order.
+        // However, we're only guaranteed that they appear in
+        // the same rankOrder. Thus we must search one of these
+        // in the other's rankOrder.
+        if (!keyEQ(specimenKeys, pattKeys)) {
+          return false;
+        }
+        return compressRecur(specimenValues, valuePatts);
+      }
+      default:
+        {
+          const matchHelper = maybeMatchHelper(patternKind);
+          if (matchHelper) {
+            if (matchHelper.compress) {
+              const subCompressedRecord = matchHelper.compress(
+                innerSpecimen,
+                innerPattern.payload,
+                compress,
+              );
+              if (subCompressedRecord === undefined) {
+                return false;
+              } else {
+                emitBinding(subCompressedRecord.compressed);
+                return true;
+              }
+            } else if (matches(innerSpecimen, innerPattern)) {
+              assert(isNonCompressingMatcher(innerPattern));
+              emitBinding(innerSpecimen);
+              return true;
+            } else {
+              return false;
+            }
+          }
+        }
+        throw Fail`unrecognized kind: ${q(patternKind)}`;
+    }
+  };
+
+  if (compressRecur(specimen, pattern)) {
+    return harden({ compressed: bindings });
+  } else {
+    return undefined;
+  }
+};
+harden(compress);
+
+/**
+ * `mustCompress` is to `compress` approximately as `mustMatch` is to `matches`.
+ * Where `compress` indicates pattern match failure by returning `undefined`,
+ * `mustCompress` indicates pattern match failure by throwing an error
+ * with a good pattern-match-failure diagnostic. Thus, like `mustMatch`,
+ * `mustCompress` has an additional optional `label` parameter to be used on
+ * the outside of that diagnostic if needed. If `mustCompress` does return
+ * normally, then the pattern match succeeded and `mustCompress` returns a
+ * valid compressed value.
+ *
+ * @type {MustCompress}
+ */
+export const mustCompress = (specimen, pattern, label = undefined) => {
+  const compressedRecord = compress(specimen, pattern);
+  if (compressedRecord !== undefined) {
+    return compressedRecord.compressed;
+  }
+  // `compress` is validating, so we don't need to redo all of `mustMatch`.
+  // We use it only to generate the error.
+  // Should only throw
+  checkMatches(specimen, pattern, assertChecker, label);
+  throw Fail`internal: ${label}: inconsistent pattern match: ${q(pattern)}`;
+};
+harden(mustCompress);
+
+/**
+ * `decompress` reverses the compression performed by `compress`
+ * or `mustCompress`, in order to recover the equivalent
+ * of the original specimen from the `bindings` array and the `pattern`.
+ *
+ * @type {Decompress}
+ */
+const decompress = (compressed, pattern) => {
+  if (isNonCompressingMatcher(pattern)) {
+    return compressed;
+  }
+
+  assert(Array.isArray(compressed));
+  passStyleOf(compressed) === 'copyArray' ||
+    Fail`Pattern ${pattern} expected bindings array: ${compressed}`;
+  let i = 0;
+  const takeBinding = () => {
+    i < compressed.length ||
+      Fail`Pattern  ${q(pattern)} expects more than ${q(
+        compressed.length,
+      )} bindings: ${compressed}`;
+    const binding = compressed[i];
+    i += 1;
+    return binding;
+  };
+  harden(takeBinding);
+
+  const decompressRecur = innerPattern => {
+    assertPattern(innerPattern);
+    if (isKey(innerPattern)) {
+      return innerPattern;
+    }
+    const patternKind = kindOf(innerPattern);
+    switch (patternKind) {
+      case undefined: {
+        throw Fail`decompress expected a pattern: ${q(innerPattern)}`;
+      }
+      case 'copyArray': {
+        return harden(innerPattern.map(p => decompressRecur(p)));
+      }
+      case 'copyRecord': {
+        const pattNames = recordNames(innerPattern);
+        const pattValues = recordValues(innerPattern, pattNames);
+        const entries = pattNames.map((name, j) => [
+          name,
+          decompressRecur(pattValues[j]),
+        ]);
+        // Reverse so printed form looks less surprising,
+        // with ascenting rather than descending property names.
+        return harden(fromEntries(entries.reverse()));
+      }
+      case 'copyMap': {
+        const {
+          payload: { keys: pattKeys, values: valuePatts },
+        } = innerPattern;
+        return makeTagged(
+          'copyMap',
+          harden({
+            keys: pattKeys,
+            values: valuePatts.map(p => decompressRecur(p)),
+          }),
+        );
+      }
+      default:
+        {
+          const matchHelper = maybeMatchHelper(patternKind);
+          if (matchHelper) {
+            if (matchHelper.decompress) {
+              const subCompressed = takeBinding();
+              return matchHelper.decompress(
+                subCompressed,
+                innerPattern.payload,
+                decompress,
+              );
+            } else {
+              assert(isNonCompressingMatcher(innerPattern));
+              return takeBinding();
+            }
+          }
+        }
+        throw Fail`unrecognized pattern kind: ${q(patternKind)} ${q(
+          innerPattern,
+        )}`;
+    }
+  };
+
+  return decompressRecur(pattern);
+};
+harden(decompress);
+
+/**
+ * `decompress` reverses the compression performed by `compress`
+ * or `mustCompress`, in order to recover the equivalent
+ * of the original specimen from `compressed` and `pattern`.
+ *
+ * @type {MustDecompress}
+ */
+export const mustDecompress = (compressed, pattern, label = undefined) => {
+  const value = decompress(compressed, pattern);
+  // `decompress` does some checking, but is not validating, so we
+  // need to do the full `mustMatch` here to validate as well as to generate
+  // the error if invalid.
+  mustMatch(value, pattern, label);
+  return value;
+};