Cairo: How do write a log function in Cairo

This is some code that is not optimized for big numbers, but currently works



%builtins output range_check



from starkware.cairo.common.serialize import serialize_word

from starkware.cairo.common.math_cmp import is_le

from starkware.cairo.common.math_cmp import is_not_zero

from starkware.cairo.common.math import assert_250_bit



func main{output_ptr : felt*, range_check_ptr}():

let (exp) = findLog(2,33)

serialize_word(exp)

return ()

end



#Get log base(target) -> say base = 2 and target = 32 then log2(32) = 5. This will give the floor for any decimal value

func findLog{range_check_ptr: felt}(base : felt, target : felt) -> (exp : felt):

return findPowLarger(base,0,1,target)

end



#Find the next value = base ^ exp such that value is not greater than target

func findPowLarger{range_check_ptr: felt}(base : felt, exp : felt, currVal : felt, target : felt) -> (exp : felt):

alloc_locals

#Getting the next power of the base on this iteration

local newVal = base * currVal

assert_250_bit(newVal)

#This handles flooring scenario

let (isLe) = is_le(newVal, target)

if isLe == 1:

return findPowLarger(base, exp + 1, newVal, target)

end

#This handles exact match for base ^ exponent = target

let (isNotEqual) = is_not_zero(newVal - target)

if isNotEqual == 0:

return (exp = exp)

end

return (exp = exp)

end

ㅤ This answer was originally posted on Triality