Fix some inconsistencies in error cost calculation

Signed-off-by: Nathan Sobo <nathan@github.com>

src/runtime/error_costs.h

@@ -0,0 +1,43 @@
+#ifndef RUNTIME_ERROR_COSTS_H_
+#define RUNTIME_ERROR_COSTS_H_
+
+#define ERROR_COST_PER_SKIPPED_TREE 10
+#define ERROR_COST_PER_SKIPPED_LINE 3
+#define ERROR_COST_PER_SKIPPED_CHAR 0
+
+typedef struct {
+  unsigned cost;
+  unsigned count;
+  unsigned depth;
+} ErrorStatus;
+
+static inline int error_status_compare(ErrorStatus a, ErrorStatus b) {
+  static unsigned ERROR_COST_THRESHOLD = 3 * ERROR_COST_PER_SKIPPED_TREE;
+  static unsigned ERROR_COUNT_THRESHOLD = 1;
+
+  // TODO remove
+  a.cost += ERROR_COST_PER_SKIPPED_TREE * a.count;
+  b.cost += ERROR_COST_PER_SKIPPED_TREE * b.count;
+
+  if ((a.count + ERROR_COUNT_THRESHOLD < b.count) ||
+      (a.count < b.count && a.cost <= b.cost)) {
+    return -1;
+  }
+
+  if ((b.count + ERROR_COUNT_THRESHOLD < a.count) ||
+      (b.count < a.count && b.cost <= a.cost)) {
+    return 1;
+  }
+
+  if (a.cost + ERROR_COST_THRESHOLD < b.cost) {
+    return -1;
+  }
+
+  if (b.cost + ERROR_COST_THRESHOLD < a.cost) {
+    return 1;
+  }
+
+  return 0;
+}
+
+#endif