[feat req] assert_almost_equals() margin relative to precision of expected number

[midrare]

I'd like a way to auto-calculate the margin for assert_almost_equals() based on lua's default tostring() decimal precision. This lets you copy and paste the result of a known good calculation and use it as the expected result for an assert.

For example:

print(3 / 7)  -- prints "0.42857142857143"
luaunit.assert_almost_equals(3 / 7, 0.42857142857143, 'default') -- passes

---

I came up with a possible implementation (posted here under public domain).

local default_margin_precision = #(tostring(math.pi)) - 1  -- sub 1 for dot char

local function calc_margin(n, prec)
    prec = (prec == nil and default_margin_precision) or prec
    assert(prec > 0, "precision must be an int greater than zero")
    local digits_over_zero = math.ceil(math.log(math.abs(n), 10))
    local digits_below_zero = prec - digits_over_zero
    return 1.0 / 10^math.min(digits_below_zero, math.max(0, prec - 1))
end

assert(calc_margin(3.4883172844444) == 0.0000000000001)
assert(calc_margin(34883.172844444) == 00000.000000001)
assert(calc_margin(0.3488317284444) == 0.0000000000001)
assert(calc_margin(348831728444.44) == 000000000000.01)
assert(calc_margin(0.0000348831728) == 0.0000000000001)

luaunit.assert_almost_equals(3/7, 0.42857142857143, calc_margin(0.42857142857143))