spiegel-keyman/common/web/input-processor/src/text/contextWindow.ts

namespace com.keyman.text {
  export class ContextWindow implements Context {
    // Used to limit the range of context replicated for use of keyboard rules within
    // the engine, as used for fat-finger prep / `Alternate` generation.
    public static readonly ENGINE_RULE_WINDOW: Configuration = {
      leftContextCodePoints: 64,
      rightContextCodePoints: 32
    };

    left: string;
    right?: string;

    startOfBuffer: boolean;
    endOfBuffer: boolean;

    casingForm?: CasingForm;

    constructor(mock: Mock, config: Configuration, layerId: string) {
      this.left = mock.getTextBeforeCaret();
      this.startOfBuffer = this.left._kmwLength() <= config.leftContextCodePoints;
      if(!this.startOfBuffer) {
        // Our custom substring version will return the last n characters if param #1 is given -n.
        this.left = this.left._kmwSubstr(-config.leftContextCodePoints);
      }

      this.right = mock.getTextAfterCaret();
      this.endOfBuffer = this.right._kmwLength() <= config.rightContextCodePoints;
      if(!this.endOfBuffer) {
        this.right = this.right._kmwSubstr(0, config.rightContextCodePoints);
      }

      this.casingForm =
        layerId == 'shift' ? 'initial' :
        layerId == 'caps' ? 'upper' :
        null;
    }

    public toMock(): Mock {
      let caretPos = this.left._kmwLength();

      return new Mock(this.left + (this.right || ""), caretPos);
    }
  }
}