From 71a5cc94a1c0fe2ce035f9c0c26ee5df8bc2953b Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Thu, 20 Jul 2017 13:13:57 -0400
Subject: Lex and parse keepHigh

---
 __tests__/lexer.test.js  | 12 ++++++++++++
 __tests__/parser.test.js | 12 ++++++++++++
 index.js                 |  2 ++
 src/lexer.js             |  1 +
 src/parser.js            | 14 ++++++++++++++
 5 files changed, 41 insertions(+)

diff --git a/__tests__/lexer.test.js b/__tests__/lexer.test.js
index 3003fb9..04a95e1 100644
--- a/__tests__/lexer.test.js
+++ b/__tests__/lexer.test.js
@@ -136,4 +136,16 @@ describe('lex', () => {
       ])
     })
   })
+
+  describe('keep', () => {
+    test('1K2d20', () => {
+      expect(lex('1K2d20')).toEqual([
+        { type: 'constant', value: 1 },
+        { type: 'K' },
+        { type: 'constant', value: 2 },
+        { type: 'd' },
+        { type: 'constant', value: 20 }
+      ])
+    })
+  })
 })
diff --git a/__tests__/parser.test.js b/__tests__/parser.test.js
index 26febef..fb554f9 100644
--- a/__tests__/parser.test.js
+++ b/__tests__/parser.test.js
@@ -127,6 +127,18 @@ describe('parse', () => {
     })
   })
 
+  it('parses dice with keep', () => {
+    expect(parse('1K2d20')).toEqual({
+      type: 'K',
+      left: { type: 'constant', value: 1 },
+      right: {
+        type: 'd',
+        left: { type: 'constant', value: 2 },
+        right: { type: 'constant', value: 20 }
+      }
+    })
+  })
+
   describe('parsing parentheses', () => {
     test('(1d6)d6', () => {
       expect(parse('(1d6)d6')).toEqual({
diff --git a/index.js b/index.js
index 3b858a2..4a58e4a 100644
--- a/index.js
+++ b/index.js
@@ -9,6 +9,8 @@ const interpret = tree => {
     return D.d(interpret(tree.left), interpret(tree.right))
   case 'E':
     return D.explode(interpret(tree.left), interpret(tree.right))
+  case 'K':
+    return D.keepHigh(interpret(tree.left), interpret(tree.right))
   case 'add':
     return D.add(interpret(tree.left), interpret(tree.right))
   case 'subtract':
diff --git a/src/lexer.js b/src/lexer.js
index 85579fb..fd0d0a3 100644
--- a/src/lexer.js
+++ b/src/lexer.js
@@ -25,6 +25,7 @@ newLexemeType('-', '-')
 newLexemeType('(', '\\(')
 newLexemeType(')', '\\)')
 newLexemeType('E', 'E')
+newLexemeType('K', 'K')
 newSkippableLexeme('whitespace', '\\s+')
 
 const lex = (expressionString) => {
diff --git a/src/parser.js b/src/parser.js
index 9479abc..5e9960a 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -6,6 +6,8 @@ let throwSyntaxError = () => {
   throw new Error('Syntax error: unexpected token')
 }
 
+const dieBindingPower = 30
+
 let newSymbol = (type, nud, lbp, led) => {
   symbols[type] = {
     type,
@@ -33,6 +35,16 @@ newSymbol('(', function(parser) {
 
 newSymbol(')')
 
+const newDieOperation = (symbol, nud = null) => {
+  newSymbol(symbol, nud, dieBindingPower, (left, parser) => {
+    return {
+      type: symbol,
+      left: left,
+      right: parser.expression(dieBindingPower - 1)
+    }
+  })
+}
+
 newSymbol('d', (parser) => {
   return {
     type: 'd',
@@ -55,6 +67,8 @@ newSymbol('E', null, 30, (left, parser) => {
   }
 })
 
+newDieOperation('K')
+
 newSymbol('+', null, 20, (left, parser) => {
   return {
     type: 'add',
-- 
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