Class: Semdiff::IdentityCompiler

Inherits:
Prism::MutationCompiler
  • Object
show all
Includes:
Prism::DSL, CompilerUtils
Defined in:
lib/semdiff/identity_compiler.rb

Overview

IdentityCompiler is a compiler that simplifies arithmetic identities for variables based on type information.

Examples:

```ruby
# Expression        Canonical Form
x + 0               x       # zero addition
x - 0               x       # zero subtraction
0 - x               -x      # unary negation
x * 1               x       # multiplicative identity
0 * x               0       # zero multiplication
x / 1               x       # division identity
x ** 1              x       # exponent identity
x ** 0              1       # zero exponent
+x                  x       # unary plus is noop
-(-x)               x       # double negation
---x                -x      # multiple negation
```

Instance Method Summary collapse

Methods included from CompilerUtils

inherit_newline

Constructor Details

#initialize(node_types) ⇒ IdentityCompiler

Returns a new instance of IdentityCompiler.



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# File 'lib/semdiff/identity_compiler.rb', line 26

def initialize(node_types)
  @node_types = node_types
  super()
end

Instance Method Details

#visit_call_node(node) ⇒ Object 



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# File 'lib/semdiff/identity_compiler.rb', line 31

def visit_call_node(node)
  lhs         = visit(node.receiver)
  arguments   = visit(node.arguments)
  rhs         = arguments&.arguments&.first
  lhs_types   = lhs && @node_types[lhs.node_id]
  rhs_types   = rhs && @node_types[rhs.node_id]
  lhs_numeric = lhs_types&.anybits?(NodeTypeFlags::NUMERIC)
  rhs_numeric = rhs_types&.anybits?(NodeTypeFlags::NUMERIC)
  lhs_integer = lhs_numeric && lhs_types.anybits?(NodeTypeFlags::INTEGER)
  rhs_integer = rhs_numeric && rhs_types.anybits?(NodeTypeFlags::INTEGER)
  result      = nil

  case node.name
  when :+
    if lhs_numeric && zero_literal?(rhs)
      # x + 0 => x
      result = lhs
    elsif rhs_numeric && zero_literal?(lhs)
      # 0 + x => x
      result = rhs
    end
  when :+@
    # + x => x
    result = lhs if lhs_numeric
  when :-
    if lhs_numeric && zero_literal?(rhs)
      # x - 0 => x
      result = lhs
    elsif rhs_numeric && zero_literal?(lhs)
      # 0 - x => -x
      result = node.copy(
        receiver: rhs,
        name: :-@,
        arguments: nil
      )
    end
  when :-@
    inner_lhs = lhs
    neg_count = 1
    loop do
      if inner_lhs.type == :parentheses_node && !inner_lhs.multiple_statements?
        # -( - x ) => x
        inner_lhs = inner_lhs.body.body.first
        next
      end
      if inner_lhs.type == :call_node && inner_lhs.name == :-@
        # ---x => -x
        inner_lhs = inner_lhs.receiver
        neg_count += 1
        next
      end
      break
    end
    result = inner_lhs if neg_count.even?
  when :*
    if lhs_numeric && one_literal?(rhs)
      # x * 1 => x
      result = lhs
    elsif rhs_numeric && one_literal?(lhs)
      # 1 * x => x
      result = rhs
    elsif (lhs_integer && zero_literal?(rhs)) ||
          (rhs_integer && zero_literal?(lhs))
      # 0 * x or x * 0 => 0
      # This works for integers, but not for Float
      # because of INFINITY or NaN instead of 0.
      result = rhs && zero_literal?(rhs) ? rhs : lhs
    end
  when :/
    # x / 1 => x
    result = lhs if lhs_numeric && one_literal?(rhs)
  when :**
    if lhs_numeric && one_literal?(rhs)
      # x ** 1 => x
      result = lhs
    elsif lhs_integer && zero_literal?(rhs)
      # x ** 0 => 1
      flags = integer_base_flag(:decimal) | Prism::NodeFlags::STATIC_LITERAL
      result = integer_node(
        source: node.send(:source),
        location: node.location,
        flags: flags,
        value: 1
      )
    end
  end

  if result
    CompilerUtils.inherit_newline(node, result)
  else
    # finalized transformations
    node.copy(receiver: lhs, arguments: arguments, block: visit(node.block))
  end
end