First few Project Euler problems solved in CoffeeScript. Comments on how to make it more beautiful (in terms of CoffeeScript, not algorithm) highly appreciated!

ProjectEuler.coffee

@_1 = ->

  # sum of multiples of 3 or 5 (not both) under 1K

  answer = 0

  for n in [1...1000]
    if n % 3 == 0 or n % 5 == 0
      answer += n

  answer

@_2 = ->

  # sum of even fibonacci numbers up to 4M

  answer = 0
  [f,s,t] = [0,1,1]
  
  while t <= 4000000
    [f,s,t] = [s,t,s+t]
    if t % 2 == 0
      answer += t

  answer

@_3 = ->

  # highest factor of a large number

  factor(600851475143).max()

@_4 = ->
  
  # highest palindromic number that is product of two 3-digit numbers

  max = 0

  isP = (n) ->
    n.toString() == n.toString().reverse()

  for x in [100..999]
    for y in [100..999]
      tmp = x*y
      if isP(tmp) and max < tmp
        max = tmp

  max

@_5 = ->

  # lowest number divisible by numbers 1..20

  answer = 1

  for prime in primes_upto 20
    tmp = prime
    while tmp < 20
      answer *= prime
      tmp *= prime

  answer

@_6 = ->

  # difference between squared sum of 1..100 and sum of squares of 1..100

  sum = (x for x in [1..100])
  sqs = (x*x for x in sum)

  sqsum = Math.pow sum.sum(), 2
  sumsqs = sqs.sum()

  answer = sqsum - sumsqs

@_7 = ->

  # 10001th prime

  prime 10001

@_8 = ->

  # highest product of 5 consecutive digits of 1000-digit number

  max = 0
  num = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"

  for i in [0..num.length-5]
    tmp = 1
    for char in num.substr i,5
      tmp *= parseInt char
    if tmp > max
      max = tmp

  max

@_9 = ->

  # pythagorean triple such that a+b+c = 1K

  for a in [1...1000]
    aa = a*a
    for b in [a...1000-a]
      bb = b*b
      c = 1000-a-b
      if aa + bb == c*c
        return a*b*c

@_10 = ->

  # sum of primes up to 2M

  primes_upto(2000000).sum()

# ==============
# helper methods
# ==============

String::reverse = ->

  # "abc" -> "cba"

  this.split('').reverse().join('')

Array::max = ->

  # [1,3,2] -> 3

  Math.max.apply Math, this

Array::min = ->

  # [1,3,2] -> 1

  Math.min.apply Math, this

Array::sum = ->

  # [1,3,5] -> 9

  a = 0

  for i in [0...this.length]
    a += this[i]

  a

@isPrime = (n) ->

  # is n prime?

  if n == 1
    return false
  if n < 4
    return true
  if n % 2 == 0
    return false
  if n < 9
    return true
  if n % 3 == 0
    return false

  r = Math.floor Math.sqrt n
  f = 5
  while f <= r
    if n % f == 0
      return false
    if n % (f+2) == 0
      return false
    f += 6
  return true

@primes_upto = (n) ->

  # all primes up to n

  (x for x in [1..n] when isPrime x)

@primes = (n) ->

  # n primes

  rank = 0
  i = 1
  ps = []

  while rank < n
    if isPrime ++i
      ps.push i
      rank++

  ps

@prime = (n) ->

  # nth prime

  rank = 1
  i = 2

  while rank < n
    if isPrime ++i
      rank++

  i

@factor = (n) ->

  # 60 -> [2,2,3,5]

  facs = []
  divisor = 2
  
  while n > 1
    while n % divisor == 0
      facs.push divisor
      n /= divisor
    divisor++

  facs