module Brakeman::CallConversionHelper

def hash_values_at hash, keys

which indicate a key could not be found.
You must check the return value for `nil`s - 
def hash_values_at hash, keys
  values = keys.map do |key|
    process_hash_access hash, key
  end
  Sexp.new(:array).concat(values).line(hash.line)
end

def join_arrays lhs, rhs, original_exp = nil

Join two array literals into one. 
def join_arrays lhs, rhs, original_exp = nil
  if array? lhs and array? rhs
    result = Sexp.new(:array)
    result.line(lhs.line || rhs.line || 1)
    result.concat lhs[1..-1]
    result.concat rhs[1..-1]
    result
  else
    original_exp
  end
end

def join_strings lhs, rhs, original_exp = nil

Join two string literals into one. 
def join_strings lhs, rhs, original_exp = nil
  if string? lhs and string? rhs
    if (lhs.value.length + rhs.value.length > STRING_LENGTH_LIMIT)
      # Avoid gigantic strings
      lhs
    else
      result = Sexp.new(:str).line(lhs.line)
      result.value = lhs.value + rhs.value
      result
    end
  elsif call? lhs and lhs.method == :+ and string? lhs.first_arg and string? rhs
    joined = join_strings lhs.first_arg, rhs
    lhs.first_arg = joined
    lhs
  elsif safe_literal? lhs or safe_literal? rhs
    safe_literal(lhs.line)
  else
    original_exp
  end
rescue Encoding::CompatibilityError => e
  # If the two strings are different encodings, we can't join them.
  Brakeman.debug e.inspect
  original_exp
end

def math_op op, lhs, rhs, original_exp = nil 

def math_op op, lhs, rhs, original_exp = nil
  if number? lhs and number? rhs
    if op == :/ and rhs.value == 0 and not lhs.value.is_a? Float
      # Avoid division by zero
      return original_exp
    else
      value = lhs.value.send(op, rhs.value)
      Sexp.new(:lit, value).line(lhs.line)
    end
  elsif call? lhs and lhs.method == :+ and number? lhs.first_arg and number? rhs
    # (x + 1) + 2 -> (x + 3)
    lhs.first_arg = Sexp.new(:lit, lhs.first_arg.value + rhs.value).line(lhs.first_arg.line)
    lhs
  elsif safe_literal? lhs or safe_literal? rhs
    safe_literal(lhs.line)
  else
    original_exp
  end
end

def process_array_access array, args, original_exp = nil

Returns the value inside the array, if possible.

Process single integer access to an array. 
def process_array_access array, args, original_exp = nil
  if args.length == 1 and integer? args.first
    index = args.first.value
    #Have to do this because first element is :array and we have to skip it
    array[1..-1][index] or original_exp
  elsif all_literals? array
    safe_literal(array.line)
  else
    original_exp
  end
end

def process_hash_access hash, index, original_exp = nil

with the given argument.
Process hash access by returning the value associated 
def process_hash_access hash, index, original_exp = nil
  if value = hash_access(hash, index)
    value # deep_clone?
  elsif all_literals? hash, :hash
    safe_literal(hash.line)
  else
    original_exp
  end
end