permute_tx: fix horrible thinko.

Map needs to be inverted, otherwise non-trivial shuffles get confused.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

permute_tx.c

@@ -45,6 +45,21 @@ static void init_map(size_t *map, size_t len)
 		map[i] = i;
 }
 
+/* This map says where things ended up, eg. 0 might be in slot 3.  we
+ * want to change it so map[0] = 3. */
+static void invert_map(size_t *map, size_t len)
+{
+	if (map) {
+		size_t i, newmap[len];
+
+		memset(newmap, 0, sizeof(newmap));
+		for (i = 0; i < len; i++) {
+			newmap[map[i]] = i;
+		}
+		memcpy(map, newmap, sizeof(newmap));
+	}
+}
+
 static bool input_better(const struct bitcoin_tx_input *a,
 			 const struct bitcoin_tx_input *b)
 {
@@ -117,6 +132,8 @@ void permute_inputs(uint64_t seed1, uint64_t seed2, uint64_t tx_num,
 		size_t r = get_next_rand(&h, &randidx) % (num_inputs - i - 1);
 		swap_inputs(inputs, map, i, i + 1 + r);
 	}
+
+	invert_map(map, num_inputs);
 }
 
 static void swap_outputs(struct bitcoin_tx_output *outputs, size_t *map,
@@ -183,4 +200,6 @@ void permute_outputs(uint64_t seed1, uint64_t seed2, size_t tx_num,
 		size_t r = get_next_rand(&h, &randidx) % (num_outputs - i - 1);
 		swap_outputs(outputs, map, i, i + 1 + r);
 	}
+
+	invert_map(map, num_outputs);
 }