Able to interpret brainfuck

2 files changed, 76 insertions, 2 deletions

diff --git a/src/brainfuck/brainfuck.cr b/src/brainfuck/brainfuck.cr
index b75ad1c..819fe8e 100644
--- a/src/brainfuck/brainfuck.cr
+++ b/src/brainfuck/brainfuck.cr
@@ -3,6 +3,9 @@ require "./*"
 module Brainfuck
   DEFAULT_TAPE_SIZE = 30_000
 
+  class MemoryError < Exception
+  end
+
   enum Opcode
     PtrRight
     PtrLeft
diff --git a/src/brainfuck/program.cr b/src/brainfuck/program.cr
index cf4a097..5994d40 100644
--- a/src/brainfuck/program.cr
+++ b/src/brainfuck/program.cr
@@ -1,14 +1,85 @@
 require "./parser.cr"
 
 class Brainfuck::Program
-  def initialize(@parser : Brainfuck::Parser)
+  @mem_size : Int32
+
+  def initialize(@parser : Parser, mem_size : Int32?)
+    @mem_size = mem_size || DEFAULT_TAPE_SIZE
   end
 
   def interpret
-    puts @parser.parse
+    code = @parser.parse
+    jumps = find_jumps(code)
+    code_ptr = 0
+
+    memory = [0_u8] * @mem_size
+    mem_ptr = 0
+
+    while code_ptr < code.size
+      case code[code_ptr]
+      when Opcode::PtrRight
+        mem_ptr += 1
+
+      when Opcode::PtrLeft
+        mem_ptr -= 1
+
+      when Opcode::Inc
+        begin
+          memory[mem_ptr] += 1_u8
+        rescue OverflowError
+          memory[mem_ptr] = 0_u8
+        end
+
+      when Opcode::Dec
+        begin
+          memory[mem_ptr] -= 1_u8
+        rescue OverflowError
+          memory[mem_ptr] = 0_u8
+        end
+
+      when Opcode::Out
+        print memory[mem_ptr].chr
+
+      when Opcode::In
+        c = STDIN.read_char
+        memory[mem_ptr] = c.nil? ? 0_u8 : c.ord.to_u8
+
+      when Opcode::LoopStart
+        code_ptr = jumps[code_ptr] if memory[mem_ptr] == 0_u8
+
+      when Opcode::LoopEnd
+        code_ptr = jumps[code_ptr] unless memory[mem_ptr] == 0_u8
+
+      end
+
+      raise MemoryError.new("Too far left") if mem_ptr < 0
+      raise MemoryError.new("Too far right") if mem_ptr >= memory.size
+
+      code_ptr += 1
+    end
   end
 
   def compile
+    raise NotImplementedError.new
+  end
+
+  # assumes valid code
+  private def find_jumps(code : Array(Opcode)) : Hash(Int32, Int32)
+    jumps = Hash(Int32, Int32).new
+    stack = [] of Int32
+
+    code.each_index do |i|
+      if code[i] == Opcode::LoopStart
+        stack.push(i)
+      end
+
+      if code[i] == Opcode::LoopEnd
+        jumps[i] = stack.pop
+        jumps[jumps[i]] = i
+      end
+    end
+
+    return jumps
   end
 end