Donchian Channels

The Donchian Channels indicator from the public library (donchian_channels), written against the current DSL.

What it plots

Three lines on price: the highest high, the lowest low, and their midpoint over period bars.

Key decisions

Full script

meta({
  shortName: "DC",
  kind: "overlay",
  dataSource: "candles",
  category: "custom",
});

dialog({
  title: "Donchian Channels Configuration",
  description: "Configure Donchian Channels settings",
  width: 400,
  height: 400,
  resizable: false,
  showEnabledCheckbox: true,
  resetToDefaults: {
    enabled: true,
    confirmationMessage: "Are you sure you want to reset all Donchian Channels settings to their default values?",
  },
});

const period = input.number({
  id: "period",
  label: "Period",
  default: 20,
  min: 1,
  step: 1,
});

const upperColor = input.color({
  id: "upperColor",
  label: "Upper Color",
  default: "#2962FF",
  allowAlpha: true,
});

const middleColor = input.color({
  id: "middleColor",
  label: "Middle Color",
  default: "#FF6D00",
  allowAlpha: true,
});

const lowerColor = input.color({
  id: "lowerColor",
  label: "Lower Color",
  default: "#2962FF",
  allowAlpha: true,
});

const lineStyle = input.lineStyle({
  id: "lineStyle",
  label: "Line Style",
  default: LineStyle.Solid,
});

const labelsEnabled = input.boolean({
  id: "labelsEnabled",
  label: "Show Labels",
  default: true,
});

warmup((w) => w.window(period));

layout(
  tab({ id: "general", title: "General" },
    section({ id: "parameters", title: "Parameters" }, period),
    section({ id: "display", title: "Display Options" }, labelsEnabled),
  ),
  tab({ id: "appearance", title: "Appearance" },
    section({ id: "lines", title: "Lines", columns: 2 }, upperColor, middleColor, lowerColor, lineStyle),
  ),
);

output.line({
  id: "upper",
  lineWidth: 1,
  colorFrom: upperColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "DC Upper",
});

output.line({
  id: "middle",
  lineWidth: 1,
  colorFrom: middleColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "DC Mid",
});

output.line({
  id: "lower",
  lineWidth: 1,
  colorFrom: lowerColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "DC Lower",
});

// onBar runs again for the forming bar on every tick. The chart rolls ctx.accum back between
// ticks; foldBar does not depend on that, so the same code is right on every runtime.
// foldBar keeps two states: `done` (as of the last closed bar) and `tip` (this bar). `step`
// builds the tip from `done` without mutating it, so a repeat of the same bar recomputes the tip
// instead of stacking on top of it. A new bar time commits the tip.
function foldBar(ctx, id, init, step) {
  const s = ctx.accum(id, () => ({ t: null, done: init, tip: init }), (s) => {
    if (s.t !== ctx.time) {
      s.done = s.tip;
      s.t = ctx.time;
    }
    s.tip = step(s.done);
    return s;
  });
  return s.tip;
}

function mean(values) {
  let sum = 0;
  for (const v of values) sum += v;
  return sum / values.length;
}

/** @param {ScriptedCtx} ctx */
export function onBar(ctx) {
  const p = ctx.params.period;

  const st = foldBar(ctx, "dc", { highs: [], lows: [] }, (prev) => ({
    highs: prev.highs.concat(ctx.high).slice(-p),
    lows: prev.lows.concat(ctx.low).slice(-p),
  }));
  if (st.highs.length < p) return;

  const highest = Math.max(...st.highs);
  const lowest = Math.min(...st.lows);
  ctx.plot("upper", highest);
  ctx.plot("lower", lowest);
  ctx.plot("middle", (highest + lowest) / 2);
}

Related

Series types for the band fill, and Warmup and memory for why a lookback needs warmup.