Tracks: Highlight more counting errors if track looks good

Normally, Tracks puts error marks on row and column clues only when one
of the existing track or no-track marks will have to be removed to
satisfy the clue.  This could lead to a situation where the player had
built a track from A to B and got neither a win nor a highlighted error
because the only error was in a row or column having too few track
segments.

This commit re-arranges the completion checking so that if there's a
complete track from A to B and no spurious track then the game will
highlight any clue that isn't matched by the actually laid track.  This
should mean that any solution with a track from A to B will either be a
win or have a highlighted error.

This should fix Android issue #266.

tracks.c

@@ -1823,7 +1823,7 @@ static int tracks_neighbour(int vertex, void *vctx)
 static bool check_completion(game_state *state, bool mark)
 {
     int w = state->p.w, h = state->p.h, x, y, i, target;
-    bool ret = true;
+    bool ret = true, pathret;
     int ntrack, nnotrack, ntrackcomplete;
     int *dsf, pathclass;
     struct findloopstate *fls;
@@ -1844,58 +1844,6 @@ static bool check_completion(game_state *state, bool mark)
         }
     }
 
-    /* A cell is 'complete', for the purposes of marking the game as
-     * finished, if it has two edges marked as TRACK. But it only has
-     * to have one edge marked as TRACK, or be filled in as trackful
-     * without any specific edges known, to count towards checking
-     * row/column clue errors. */
-    for (x = 0; x < w; x++) {
-        target = state->numbers->numbers[x];
-        ntrack = nnotrack = ntrackcomplete = 0;
-        for (y = 0; y < h; y++) {
-            if (S_E_COUNT(state, x, y, E_TRACK) > 0 ||
-                state->sflags[y*w+x] & S_TRACK)
-                ntrack++;
-            if (S_E_COUNT(state, x, y, E_TRACK) == 2)
-                ntrackcomplete++;
-            if (state->sflags[y*w+x] & S_NOTRACK)
-                nnotrack++;
-        }
-        if (mark) {
-            if (ntrack > target || nnotrack > (h-target)) {
-                debug(("col %d error: target %d, track %d, notrack %d",
-                       x, target, ntrack, nnotrack));
-                state->num_errors[x] = 1;
-                ret = false;
-            }
-        }
-        if (ntrackcomplete != target)
-            ret = false;
-    }
-    for (y = 0; y < h; y++) {
-        target = state->numbers->numbers[w+y];
-        ntrack = nnotrack = ntrackcomplete = 0;
-        for (x = 0; x < w; x++) {
-            if (S_E_COUNT(state, x, y, E_TRACK) > 0 ||
-                state->sflags[y*w+x] & S_TRACK)
-                ntrack++;
-            if (S_E_COUNT(state, x, y, E_TRACK) == 2)
-                ntrackcomplete++;
-            if (state->sflags[y*w+x] & S_NOTRACK)
-                nnotrack++;
-        }
-        if (mark) {
-            if (ntrack > target || nnotrack > (w-target)) {
-                debug(("row %d error: target %d, track %d, notrack %d",
-                       y, target, ntrack, nnotrack));
-                state->num_errors[w+y] = 1;
-                ret = false;
-            }
-        }
-        if (ntrackcomplete != target)
-            ret = false;
-    }
-
     dsf = snewn(w*h, int);
     dsf_init(dsf, w*h);
 
@@ -1949,6 +1897,70 @@ static bool check_completion(game_state *state, bool mark)
         }
     }
 
+    /*
+     * A cell is 'complete', for the purposes of marking the game as
+     * finished, if it has two edges marked as TRACK. But it only has
+     * to have one edge marked as TRACK, or be filled in as trackful
+     * without any specific edges known, to count towards checking
+     * row/column clue errors.
+     *
+     * This changes if we haven't found any other errors by this
+     * point, so the player has constructed a route from A to B.  In
+     * that case, we highlight any row/column where the actually laid
+     * tracks don't match the clue.
+     */
+    pathret = ret; /* Do we have a plausible solution so far? */
+    for (x = 0; x < w; x++) {
+        target = state->numbers->numbers[x];
+        ntrack = nnotrack = ntrackcomplete = 0;
+        for (y = 0; y < h; y++) {
+            if (S_E_COUNT(state, x, y, E_TRACK) > 0 ||
+                state->sflags[y*w+x] & S_TRACK)
+                ntrack++;
+            if (S_E_COUNT(state, x, y, E_TRACK) == 2)
+                ntrackcomplete++;
+            if (state->sflags[y*w+x] & S_NOTRACK)
+                nnotrack++;
+        }
+        if (mark) {
+            if (ntrack > target || nnotrack > (h-target) ||
+                (pathret && ntrackcomplete != target)) {
+                debug(("col %d error: target %d, track %d, notrack %d, "
+                       "pathret %d, trackcomplete %d",
+                       x, target, ntrack, nnotrack, pathret, ntrackcomplete));
+                state->num_errors[x] = 1;
+                ret = false;
+            }
+        }
+        if (ntrackcomplete != target)
+            ret = false;
+    }
+    for (y = 0; y < h; y++) {
+        target = state->numbers->numbers[w+y];
+        ntrack = nnotrack = ntrackcomplete = 0;
+        for (x = 0; x < w; x++) {
+            if (S_E_COUNT(state, x, y, E_TRACK) > 0 ||
+                state->sflags[y*w+x] & S_TRACK)
+                ntrack++;
+            if (S_E_COUNT(state, x, y, E_TRACK) == 2)
+                ntrackcomplete++;
+            if (state->sflags[y*w+x] & S_NOTRACK)
+                nnotrack++;
+        }
+        if (mark) {
+            if (ntrack > target || nnotrack > (w-target) ||
+                (pathret && ntrackcomplete != target)) {
+                debug(("row %d error: target %d, track %d, notrack %d, "
+                       "pathret %d, trackcomplete %d",
+                       y, target, ntrack, nnotrack, pathret, ntrackcomplete));
+                state->num_errors[w+y] = 1;
+                ret = false;
+            }
+        }
+        if (ntrackcomplete != target)
+            ret = false;
+    }
+
     if (mark)
         state->completed = ret;
     sfree(dsf);