1 files changed, 93 insertions, 0 deletions
diff --git a/src/impl/blc2.c b/src/impl/blc2.c
index e69de29..3cf7ff8 100644
--- a/src/impl/blc2.c
+++ b/src/impl/blc2.c
@@ -0,0 +1,93 @@
+// Copyright (c) 2024, Marvin Borner <dev@marvinborner.de>
+// SPDX-License-Identifier: MIT
+
+#include <log.h>
+#include <impl.h>
+#include <stdint.h>
+
+// haskell by John Tromp
+static void levenshtein_encode(size_t n, FILE *fp)
+{
+	if (n == 0) {
+		fprintf(fp, "10");
+		return;
+	}
+
+	fprintf(fp, "1");
+
+	size_t length = (8 * sizeof(n)) - __builtin_clzll(n);
+	levenshtein_encode(length - 1, fp);
+
+	if (length == 1)
+		return;
+
+	for (size_t i = 1 << (length - 2); i != 0; i >>= 1)
+		fprintf(fp, "%d", (i & n) != 0);
+}
+
+static void encode(Term *term, FILE *fp)
+{
+	switch (term->type) {
+	case ABS:
+		fprintf(fp, "00");
+		encode(term->u.abs.body, fp);
+		break;
+	case APP:
+		fprintf(fp, "01");
+		encode(term->u.app.lhs, fp);
+		encode(term->u.app.rhs, fp);
+		break;
+	case IDX:
+		levenshtein_encode(term->u.index, fp);
+		break;
+	default:
+		fatal("invalid type %d\n", term->type);
+	}
+}
+
+// by John Tromp
+static size_t levenshtein_decode(FILE *fp)
+{
+	if (getc(fp) == '0')
+		return 0;
+	size_t x, l = levenshtein_decode(fp);
+	for (x = 1; l--;)
+		x = 2 * x + (getc(fp) != '0');
+	return x;
+}
+
+static Term *decode(FILE *fp)
+{
+	Term *res = 0;
+
+	char a = getc(fp);
+
+	if (a == '0') {
+		char b = getc(fp);
+		if (b == '0') {
+			res = term_new(ABS);
+			res->u.abs.body = decode(fp);
+		} else if (b == '1') {
+			res = term_new(APP);
+			res->u.app.lhs = decode(fp);
+			res->u.app.rhs = decode(fp);
+		}
+	} else if (a == '1') {
+		res = term_new(IDX);
+		res->u.index = levenshtein_decode(fp);
+	}
+
+	if (!res)
+		fatal("invalid parsing state!\n");
+
+	return res;
+}
+
+Impl impl_blc2(void)
+{
+	return (Impl){
+		.name = "blc2",
+		.encode = encode,
+		.decode = decode,
+	};
+}