Review response to #248 (comment)

Directly use the `WDL_mu` stddev formula `2 / (ln(1/W-1) + ln(1/L-1))` to calculate the interval around the eval line.

files/src/renderer/50_table.js

@@ -29,10 +29,26 @@ const table_prototype = {
  let info = SortedMoveInfoFromTable(this)[0];
  if (info && !info.__ghost && info.__touched && (this.nodes > 1 || this.limit === 1)) {
  let return_cp = ((info.board.active === "b") ? (-info.cp) : (info.cp));
- let return_drawrate = ((info.wdl === null) ? null : (info.wdl[1] / 1000.0));
+ let return_sharpness_50confidence = null;
+ if (info.wdl !== null) {
+ if ((info.wdl[0] > 0) && (info.wdl[2] > 0)) {
+ // Reference: https://github.com/LeelaChessZero/lc0/blob/076299b1f1ca21993b2c5e82ab3e80edb5367057/src/mcts/search.cc#L237-L239
+ let sharpness_a = Math.log(1000.0 / info.wdl[0] - 1.0)
+ let sharpness_b = Math.log(1000.0 / info.wdl[2] - 1.0)
+ let sharpness_sigma = 2 / Math.max(sharpness_a + sharpness_b, 0.0);
+
+ const z_50pct = 0.6744897502; // ± sigma × 0.67... is the 50% confidence interval
+ return_sharpness_50confidence = sharpness_sigma * z_50pct;
+ } else {
+ // Safeguard against numerical issues: https://github.com/LeelaChessZero/lc0/blob/076299b1f1ca21993b2c5e82ab3e80edb5367057/src/mcts/search.cc#L232-L236
+ let decisive_pct = 1.0 - info.wdl[1] / 1000.0;
+ return_sharpness_50confidence = Math.log(2.0) / Math.log( 2.0 / decisive_pct - 1.0 );
+ }
+ }
+
  return {
  'cp': return_cp,
- 'drawrate': return_drawrate
+ 'sharpness_50confidence': return_sharpness_50confidence
  };
  } else {
  return null;
@@ -47,7 +63,8 @@ const table_prototype = {
  if (this.graph_y_version !== this.version) {
  let engine_info_graph_details = this.get_cp_details();
  if (engine_info_graph_details !== null) {
- this.graph_y_drawishness = engine_info_graph_details.drawrate;
+ let radius_50pct = engine_info_graph_details.sharpness_50confidence;
+ this.graph_y_drawishness = 1 / (1 + Math.pow(0.5, 1 / radius_50pct)) - 1 / (1 + Math.pow(0.5, -1 / radius_50pct));
 
  let cp = engine_info_graph_details.cp;
  this.graph_y = 1 / (1 + Math.pow(0.5, cp / 100));