--[[ Expression Language 1, Evaluator Module
     CSci 450: Organization of Programming Languages, Fall 2016
     H. Conrad Cunningham, Professor
     Computer and Information Science
     University of Mississippi

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2016-09-13: Separated as module from Assignment #1 code, added Nop
2016-09-16: Changed message on NOP
2016-09-21: Removed Min, Max, Set, Let for new base after HW1

This module encapsulates the Expression Evaluator.

--]]

--[[ Module names now global
local UTILITIES   = "utilities"
local ABS_SYN     = "abs_syn_01"
local ENVIRONMENT = "environment"
local VALUES      = "values_01"
--]]

-- Import Utilities module

local util = require(UTILITIES)

local treeConcat, printTree, show_data =
   util.treeConcat, util.printTree, util.show_data

-- Import Abstract Syntax module

local absyn = require(ABS_SYNTAX)

local exprToString = absyn.exprToString
local getArg       = absyn.getArg
local Nop, isNop   = absyn.Nop, absyn.isNop
local Int, isInt   = absyn.Int, absyn.isInt
local Var, isVar   = absyn.Var, absyn.isVar
local Neg, isNeg   = absyn.Neg, absyn.isNeg
local Add, isAdd   = absyn.Add, absyn.isAdd
local Sub, isSub   = absyn.Sub, absyn.isSub
local Mul, isMul   = absyn.Mul, absyn.isMul
local Div, isDiv   = absyn.Div, absyn.isDiv
local Eq,  isEq    = absyn.Eq,  absyn.isEq
local Ne,  isNe    = absyn.Ne,  absyn.isNe
local Lt,  isLt    = absyn.Lt,  absyn.isLt
local Gt,  isGt    = absyn.Gt,  absyn.isGt 
local Le,  isLe    = absyn.Le,  absyn.isLe 
local Ge,  isGe    = absyn.Ge,  absyn.isGe 
local If,  isIf    = absyn.If,  absyn.isIf 

-- Import Values module
 
local values = require(VALUES)

local FALSEVAL,   TRUEVAL = values.FALSEVAL, values.TRUEVAL
local isFalse,    isTrue  = values.isFalse,  values.isTrue
local asBoolVal           = values.asBoolVal
local valToString         = values.valToString

-- Import Environment module

local environ = require(ENVIRONMENT)

local newEnv      = environ.newEnv
local findBinding = environ.findBinding
local assign      = environ.assign
local setVal      = environ.setVal
local getVal      = environ.getVal
local dumpEnv     = environ.dumpEnv 

-- Public function "eval" evaluates Expression "e" in environment
-- "env" and returns the resulting value.

local function eval(e, env)
   print("DEBUG: Calling eval with " .. exprToString(e))
   if isInt(e) then
      return getArg(e,1)
   elseif isVar(e) then
      return getVal(getArg(e,1), env)
   elseif isNeg(e) then
      return - eval(getArg(e,1), env)
   elseif isAdd(e) then
      return eval(getArg(e,1), env) + eval(getArg(e,2), env)
   elseif isSub(e) then
      return eval(getArg(e,1), env) - eval(getArg(e,2), env)
   elseif isMul(e) then -- implement for HW1
      return eval(getArg(e,1), env) * eval(getArg(e,2), env)
   elseif isDiv(e) then -- implement for HW1
      return eval(getArg(e,1), env) / eval(getArg(e,2), env)
   elseif isEq(e) then
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left == right)
   elseif isNe(e) then -- implement for HW1
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left ~= right)
   elseif isLt(e) then -- implement for HW1
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left < right)
   elseif isGt(e) then -- implement for HW1
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left > right)
   elseif isLe(e) then -- implement for HW1
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left <= right)
   elseif isGe(e) then -- implement for HW1
      local left  = eval(getArg(e,1), env)
      local right = eval(getArg(e,2), env)
      return asBoolVal(left >= right)
   elseif isIf(e) then -- implement for HW1
      local cond = eval(getArg(e,1), env)
      if isTrue(cond) then
         return eval(getArg(e,2), env)
      elseif isFalse(cond) then
         return eval(getArg(e,3), env)
      else
         error("If condition neither true nor false for "
	       .. show_data(e))
      end
   elseif isNop(e) then
      local str = exprToString(e)
      print("Warning: Evaluating no operation " .. str)
      return 0, str
   else
      error("Invalid expression in eval: " .. show_data(e))
   end
end


-- MODULE EXPORT LIST

return {
   eval  = eval
}