A program encoding some of the information in http://www.forbes.com/sites/moneybuilder/2013/01/05/updated-2013-federal-income-tax-brackets-and-marginal-rates/

marginal.go

// The curse of a programmer; if you read articles with data,
// you continuously fight the temptation to write a program to
// play with that data.  In this case, I failed.

package main

import (
	"fmt"
	"math"
)

// Information from:
//
//     http://www.forbes.com/sites/moneybuilder/2013/01/05/
//         updated-2013-federal-income-tax-brackets-and-marginal-rates/
//
//   Percent     margins
// ---------     -----------------------------
//     10%       [0, 8925]
//     15%       [8925, 36250]
//     25%       [36250, 87850]
//     28%       [87850, 183250]
//     33%       [183250, 398350]
//     35%       [398350, 400000]
//     39.6%     [400000, inf.0]
//
// I want a program that takes my income and presents the marginal
// taxes on that income.

// Let's first represent the table above as a data structure.  Let's
// call it a tax bracket.
type TaxBracket struct {
	tax_percent  float64
	margin_start float64
	margin_end   float64
}

// Given an income, we can compute the tax for that bracket.
func (t TaxBracket) GetTax(income float64) float64 {
	if t.margin_start <= income {
		return t.tax_percent * (math.Min(income, t.margin_end) - t.margin_start) / 100.0
	}
	return 0
}

// With the data definition out of the way, we can encode the table
// as a slice of these brackets:
var TAX_BRACKETS_2013 = []TaxBracket{
	{10, 0, 8925},
	{15, 8925, 36250},
	{25, 36250, 87850},
	{28, 87850, 183250},
	{33, 183250, 398350},
	{35, 398350, 400000},
	{39.6, 400000, math.Inf(1)}}

// Now we can write a function to compute, using that table.  It's just a matter
// of asking each tax bracket to get the appropriate taxes, and accumulate the
// total:
func ComputeMarginalTaxes(income float64) float64 {
	total := 0.0
	for _, tax_bracket := range TAX_BRACKETS_2013 {
		total += tax_bracket.GetTax(income)
	}
	return total
}

func main() {
	// Let's use this to check the tax rate at $100000.  According to
	// the article, we'd owe $21293.  Let's see that we do.
	fmt.Println(ComputeMarginalTaxes(100000))
}