let testCases = [
  [[1,2,3,4,4,5,1,4,2], 3],
  [[3,4,4,2], 2],
  [[,2,3,4,1], 1],
  [[9,1,2], 2],
  [[], 0],
  [[undefined, undefined], 0],
  [null, 0],
];

//
testCases.forEach(testRunner(build));
//

function build(terrain) {
  terrain = terrain || [];
  let castleCount = terrain.reduce(([castleCount, sidecar], current) => {
    switch (sidecar.length) {
      case 0: {
        return [castleCount, [current]];
      };

      case 1: {
        let [lastEntry] = sidecar;
        return lastEntry == current
          ? [castleCount, sidecar]
          : [castleCount, [...sidecar, current]];
      };

      case 2: {
        /**
         * [1 , 9]
         * [9,  1]
         */
        let [start, middle] = sidecar;
        let isValley = (middle < start) && (middle < current);
        let isPeak   = (middle > start) && (middle > current);
        return (isPeak || isValley)
          ? [castleCount+1, [current]]
          : [castleCount, sidecar];
      }
    }
  }, [0, []])[0];
  return Boolean(terrain[0])
    ? castleCount+1
    : castleCount;
}

function testRunner(testFunc) {
  return ([input, expectedOutput]) => {
    let actualOutput = build(input);
    if(actualOutput === expectedOutput) {
      console.log("Test passed.");
    } else {
      console.log("Test failed.")
    }
    console.log("input: ", input);
    console.log("expectedOutput: ", expectedOutput);
    console.log("actualOutput: ", actualOutput);
    console.log("===========");
  }
}