Add remaining time estimate to Ninja status.

doc/manual.asciidoc

@@ -205,6 +205,7 @@ Several placeholders are available:
 `%c`:: Current rate of finished edges per second (average over builds
 specified by `-j` or its default)
 `%e`:: Elapsed time in seconds.  _(Available since Ninja 1.2.)_
+`%h`:: Estimated time remaining.  _(Available since Ninja 1.10.)_
 `%%`:: A plain `%` character.
 
 The default progress status is `"[%f/%t] "` (note the trailing space

src/build.cc

@@ -274,6 +274,28 @@ string BuildStatus::FormatProgressStatus(
         break;
       }
 
+      case 'h': {
+        double ratio_finished = double(finished_edges_) / total_edges_;
+        if (overall_rate_.Elapsed() < 1.0 || ratio_finished < 0.01) {
+          snprintf(buf, sizeof(buf), "--m --s");
+        } else {
+          double time_remaining =
+              (1.0 / ratio_finished - 1) * overall_rate_.Elapsed();
+          int hours = int(time_remaining) / 3600;
+          int minutes = (int(time_remaining) % 3600) / 60;
+          int seconds = int(time_remaining) % 60;
+          if (hours > 0) {
+            snprintf(buf, sizeof(buf), "%dh %dm", hours, minutes);
+          } else if (minutes > 0) {
+            snprintf(buf, sizeof(buf), "%dm %ds", minutes, seconds);
+          } else {
+            snprintf(buf, sizeof(buf), "%ds", seconds);
+          }
+        }
+        out += buf;
+        break;
+      }
+
       default:
         Fatal("unknown placeholder '%%%c' in $NINJA_STATUS", *s);
         return "";