Parse negative bonuses

6 files changed, 68 insertions, 1 deletions

diff --git a/README.md b/README.md
index 86db32b..e868675 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,7 @@ Semantics are defined in terms of the `pool` function.
 * `D - E` is equivalent to `D + (-E)`.
 * `D+E` is the additive bonus operation. For each die in `D`'s pool, the die is
   rolled and `roll(E)` is added to its result.
+* `D-E` is equivalent to `D+(-E)`.
 * `DEF` (here `E` is the literal symbol `E`, `D` and `F` are dice expressions)
   is an "exploding die." First `D` is rolled.  Now each die in the dice pool
   generated by `F` is rolled repeatedly until it rolls something less than the
diff --git a/__tests__/dice.test.js b/__tests__/dice.test.js
index 3073fee..6ff86e7 100644
--- a/__tests__/dice.test.js
+++ b/__tests__/dice.test.js
@@ -8,7 +8,8 @@ const {
   explode,
   keepHigh,
   keepLow,
-  bonusAdd
+  bonusAdd,
+  bonusSubtract
 } = require('../src/dice.js')
 
 const defaultNumberRolls = 500
@@ -493,3 +494,43 @@ describe('bonusAdd', () => {
     })
   })
 })
+
+describe('bonusSubtract', () => {
+  describe('1d20-3', () => {
+    const die = bonusSubtract(d(constant(1), constant(20)), constant(3))
+    testDie(die, {
+      diceCount: 1,
+      average: {
+        average: 7.5
+      },
+      variance: {
+        variance: 33.25
+      },
+      bounds: {
+        low: -2,
+        high: 17,
+        expectLow: true,
+        expectHigh: true
+      }
+    })
+  })
+
+  describe('3d4-1', () => {
+    const die = bonusSubtract(d(constant(3), constant(4)), constant(1))
+    testDie(die, {
+      diceCount: 3,
+      average: {
+        average: 4.5
+      },
+      variance: {
+        variance: 3.75
+      },
+      bounds: {
+        low: 0,
+        high: 9,
+        expectLow: true,
+        expectHigh: true
+      }
+    })
+  })
+})
diff --git a/__tests__/parser.test.js b/__tests__/parser.test.js
index 873972b..54e1da7 100644
--- a/__tests__/parser.test.js
+++ b/__tests__/parser.test.js
@@ -163,6 +163,18 @@ describe('parse', () => {
     })
   })
 
+  it('parses negative bonuses', () => {
+    expect(parse('3d4-1')).toEqual({
+      type: 'bonusSubtract',
+      left: {
+        type: 'd',
+        left: { type: 'constant', value: 3 },
+        right: { type: 'constant', value: 4 }
+      },
+      right: { type: 'constant', value: 1 }
+    })
+  })
+
   describe('parsing parentheses', () => {
     test('(1d6)d6', () => {
       expect(parse('(1d6)d6')).toEqual({
diff --git a/index.js b/index.js
index 980c6b4..895656d 100644
--- a/index.js
+++ b/index.js
@@ -21,6 +21,8 @@ const interpret = tree => {
     return D.negative(interpret(tree.value))
   case 'bonusAdd':
     return D.bonusAdd(interpret(tree.left), interpret(tree.right))
+  case 'bonusSubtract':
+    return D.bonusSubtract(interpret(tree.left), interpret(tree.right))
   }
 }
 
diff --git a/src/dice.js b/src/dice.js
index becf7dc..58467c4 100644
--- a/src/dice.js
+++ b/src/dice.js
@@ -102,6 +102,15 @@ const bonusAdd = (die1, die2) => {
   }
 }
 
+const bonusSubtract = (die1, die2) => {
+  const negative2 = negative(die2)
+  return () => {
+    return die1().map(die => {
+      return () => die() + roll(negative2)
+    })
+  }
+}
+
 exports.pool = pool
 exports.roll = roll
 exports.constant = constant
@@ -113,3 +122,4 @@ exports.explode = explode
 exports.keepHigh = keepHigh
 exports.keepLow = keepLow
 exports.bonusAdd = bonusAdd
+exports.bonusSubtract = bonusSubtract
diff --git a/src/parser.js b/src/parser.js
index 48de2ab..244cdd1 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -66,6 +66,7 @@ newDieOperation('k')
 
 newInfix('bigPlus', 20, { type: 'add' })
 newInfix('plus', 20, { type: 'bonusAdd' })
+newInfix('minus', 20, { type: 'bonusSubtract' })
 newInfix('bigMinus', 20, { type: 'subtract' })
 newSymbol('minus', (parser) => {
   return {