From a7b971ef64f09a21902f34b6078b01d78dcf73e7 Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Wed, 30 Aug 2017 16:36:40 -0400
Subject: Implement threshold under

---
 __tests__/dice.test.js   | 23 ++++++++++++++++++++++-
 __tests__/lexer.test.js  | 10 ++++++++++
 __tests__/parser.test.js | 10 ++++++++++
 index.js                 |  2 ++
 src/dice.js              | 16 ++++++++++++++++
 src/lexer.js             |  1 +
 src/parser.js            |  1 +
 7 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/__tests__/dice.test.js b/__tests__/dice.test.js
index 272f436..fb8a155 100644
--- a/__tests__/dice.test.js
+++ b/__tests__/dice.test.js
@@ -13,7 +13,8 @@ const {
   keepLow,
   again,
   againUnder,
-  threshold
+  threshold,
+  thresholdLow
 } = require('../src/dice.js')
 
 const defaultNumberRolls = 500
@@ -625,4 +626,24 @@ describe('threshold', () => {
       }
     })
   })
+
+  describe('4t3d10', () => {
+    const die = thresholdLow(constant(4), d(constant(3), constant(10)))
+
+    testDie(die, {
+      diceCount: 3,
+      average: {
+        average: 1.2
+      },
+      variance: {
+        variance: 0.72
+      },
+      bounds: {
+        low: 0,
+        high: 3,
+        expectLow: true,
+        expectHigh: true
+      }
+    })
+  })
 })
diff --git a/__tests__/lexer.test.js b/__tests__/lexer.test.js
index 25ef816..b51c0f9 100644
--- a/__tests__/lexer.test.js
+++ b/__tests__/lexer.test.js
@@ -225,5 +225,15 @@ describe('lex', () => {
         { type: 'constant', value: 10 }
       ])
     })
+
+    test('8t3d10', () => {
+      expect(lex('8t3d10')).toEqual([
+        { type: 'constant', value: 8 },
+        { type: 't' },
+        { type: 'constant', value: 3 },
+        { type: 'd' },
+        { type: 'constant', value: 10 }
+      ])
+    })
   })
 })
diff --git a/__tests__/parser.test.js b/__tests__/parser.test.js
index ad67fc4..6e575b6 100644
--- a/__tests__/parser.test.js
+++ b/__tests__/parser.test.js
@@ -241,6 +241,16 @@ describe('parse', () => {
         right: { type: 'constant', value: 8 }
       }
     })
+
+    expect(parse('7t4d8')).toEqual({
+      type: 't',
+      left: { type: 'constant', value: 7 },
+      right: {
+        type: 'd',
+        left: { type: 'constant', value: 4 },
+        right: { type: 'constant', value: 8 }
+      }
+    })
   })
 
   describe('parsing parentheses', () => {
diff --git a/index.js b/index.js
index 740b96e..6dd9a5f 100644
--- a/index.js
+++ b/index.js
@@ -21,6 +21,8 @@ const interpret = tree => {
     return D.againUnder(interpret(tree.left), interpret(tree.right))
   case 'T':
     return D.threshold(interpret(tree.left), interpret(tree.right))
+  case 't':
+    return D.thresholdLow(interpret(tree.left), interpret(tree.right))
   case 'add':
     return D.add(interpret(tree.left), interpret(tree.right))
   case 'subtract':
diff --git a/src/dice.js b/src/dice.js
index e580c9d..53d337d 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -214,6 +214,21 @@ const threshold = (die1, die2) => {
   }
 }
 
+const thresholdLow = (die1, die2) => {
+  return () => {
+    const cutoff = roll(die1)
+
+    return die2().map(die => {
+      return () => {
+        if (die() <= cutoff) {
+          return 1
+        } else {
+          return 0
+        }
+      }
+    })
+  }
+}
 
 exports.pool = pool
 exports.roll = roll
@@ -231,3 +246,4 @@ exports.keepLow = keepLow
 exports.again = again
 exports.againUnder = againUnder
 exports.threshold = threshold
+exports.thresholdLow = thresholdLow
diff --git a/src/lexer.js b/src/lexer.js
index f33dc87..ba3d3a5 100644
--- a/src/lexer.js
+++ b/src/lexer.js
@@ -29,6 +29,7 @@ newLexemeType('k', 'k')
 newLexemeType('A', 'A')
 newLexemeType('a', 'a')
 newLexemeType('T', 'T')
+newLexemeType('t', 't')
 
 const lex = (expressionString) => {
   let lexemes = []
diff --git a/src/parser.js b/src/parser.js
index f8e2a5b..adb4b9b 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -67,6 +67,7 @@ newDieOperation('k')
 newDieOperation('A')
 newDieOperation('a')
 newDieOperation('T')
+newDieOperation('t')
 
 newInfix('bigPlus', 20, { type: 'add' })
 newInfix('bigMinus', 20, { type: 'subtract' })
-- 
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