Task: Straddling checkerboard
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace StraddlingCheckerboard
{
    class Program
    {
        public readonly static IReadOnlyDictionary<char, string> val2Key;
        public readonly static IReadOnlyDictionary<string, char> key2Val;

        static Program()
        {
            val2Key = new Dictionary<char, string> {
                {'A',"30"},  {'B',"31"}, {'C',"32"},  {'D',"33"},  {'E',"5"},   {'F',"34"},  {'G',"35"},
                {'H',"0"},   {'I',"36"}, {'J',"37"},  {'K',"38"},  {'L',"2"},   {'M',"4"},   {'.',"78"},
                {'N',"39"},  {'/',"79"}, {'O',"1"},   {'0',"790"}, {'P',"70"},  {'1',"791"}, {'Q',"71"},
                {'2',"792"}, {'R',"8"},  {'3',"793"}, {'S',"6"},   {'4',"794"}, {'T',"9"},   {'5',"795"},
                {'U',"72"},  {'6',"796"},{'V',"73"},  {'7',"797"}, {'W',"74"},  {'8',"798"}, {'X',"75"},
                {'9',"799"}, {'Y',"76"}, {'Z',"77"}};

            key2Val = val2Key.ToDictionary(kv => kv.Value, kv => kv.Key);
        }

        public static string Encode(string s)
        {
            return string.Concat(s.ToUpper().ToCharArray()
                .Where(c => val2Key.ContainsKey(c)).Select(c => val2Key[c]));
        }

        public static string Decode(string s)
        {
            return string.Concat(Regex.Matches(s, "79.|7.|3.|.").Cast<Match>()
                .Where(m => key2Val.ContainsKey(m.Value)).Select(m => key2Val[m.Value]));
        }

        static void Main(string[] args)
        {
            var enc = Encode("One night-it was on the twentieth of March, 1888-I was returning");
            Console.WriteLine(enc);
            Console.WriteLine(Decode(enc));

            Console.ReadLine();
        }
    }
}

Go: package main

import (
    "fmt"
    "strings"
)

func main() {
    key := `
 8752390146
 ET AON RIS
5BC/FGHJKLM
0PQD.VWXYZU`
    p := "you have put on 7.5 pounds since I saw you."
    fmt.Println(p)
    c := enc(key, p)
    fmt.Println(c)
    fmt.Println(dec(key, c))
}

func enc(bd, pt string) (ct string) {
    enc := make(map[byte]string)
    row := strings.Split(bd, "\n")[1:]
    r2d := row[2][:1]
    r3d := row[3][:1]
    for col := 1; col <= 10; col++ {
        d := string(row[0][col])
        enc[row[1][col]] = d
        enc[row[2][col]] = r2d+d
        enc[row[3][col]] = r3d+d
    }
    num := enc['/']
    delete(enc, '/')
    delete(enc, ' ')
    for i := 0; i < len(pt); i++ {
        if c := pt[i]; c <= '9' && c >= '0' {
            ct += num + string(c)
        } else {
            if c <= 'z' && c >= 'a' {
                c -= 'a'-'A'
            }
            ct += enc[c]
        }
    }
    return
}

func dec(bd, ct string) (pt string) {
    row := strings.Split(bd, "\n")[1:]
    var cx [10]int
    for i := 1; i <= 10; i++ {
        cx[row[0][i]-'0'] = i
    }
    r2d := row[2][0]-'0'
    r3d := row[3][0]-'0'
    for i := 0; i < len(ct); i++ {
        var r int
        switch d := ct[i]-'0'; d {
        case r2d:
            r = 2
        case r3d:
            r = 3
        default:
            pt += string(row[1][cx[d]])
            continue
        }
        i++
        if b := row[r][cx[ct[i]-'0']]; b == '/' {
            i++
            pt += string(ct[i])
        } else {
            pt += string(b)
        }
    }
    return
}