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// Copyright (c) 2024, Marvin Borner <dev@marvinborner.de>
// SPDX-License-Identifier: MIT
// only works for closed terms!
// technique proposed by Mateusz Naściszewski (afaik)
// implemented by John Tromp in Haskell
#include <log.h>
#include <impl.h>
#include <stdint.h>
static void encode_index(size_t n, size_t i, FILE *fp)
{
if (n == 0)
fatal("invalid index (must be closed)\n");
if (i == 0) {
fprintf(fp, "%s", n == 1 ? "1" : "10");
} else {
fprintf(fp, "1");
encode_index(n - 1, i - 1, fp);
}
}
static void encode_level(size_t n, Term *term, FILE *fp)
{
switch (term->type) {
case ABS:
fprintf(fp, "%s", n == 0 ? "0" : "00");
encode_level(n + 1, term->u.abs.body, fp);
break;
case APP:
fprintf(fp, "%s", n == 0 ? "1" : "01");
encode_level(n, term->u.app.lhs, fp);
encode_level(n, term->u.app.rhs, fp);
break;
case IDX:
encode_index(n, term->u.index, fp);
break;
default:
fatal("invalid type %d\n", term->type);
}
}
static void encode(Term *term, FILE *fp)
{
encode_level(0, term, fp);
}
static Term *decode_level(size_t n, FILE *fp)
{
Term *res = 0;
char a = getc(fp);
if (a == '0' && n == 0) {
res = term_new(ABS);
res->u.abs.body = decode_level(n + 1, fp);
} else if (a == '0') {
char b = getc(fp);
if (b == '0') {
res = term_new(ABS);
res->u.abs.body = decode_level(n + 1, fp);
} else if (b == '1') {
res = term_new(APP);
res->u.app.lhs = decode_level(n, fp);
res->u.app.rhs = decode_level(n, fp);
}
} else if (a == '1' && n == 0) {
res = term_new(APP);
res->u.app.lhs = decode_level(n, fp);
res->u.app.rhs = decode_level(n, fp);
} else if (a == '1') {
res = term_new(IDX);
res->u.index = 0;
while (--n != 0 && getc(fp) == '1')
res->u.index++;
}
if (!res)
fatal("invalid parsing state!\n");
return res;
}
static Term *decode(FILE *fp)
{
return decode_level(0, fp);
}
Impl impl_closed(void)
{
return (Impl){
.name = "closed",
.encode = encode,
.decode = decode,
};
}
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