Task: Stable marriage problem
C: #include <stdio.h>

int verbose = 0;
enum {
	clown = -1,
	abe, bob, col, dan, ed, fred, gav, hal, ian, jon,
	abi, bea, cath, dee, eve, fay, gay, hope, ivy, jan,
};
const char *name[] = {
	"Abe", "Bob", "Col",  "Dan", "Ed",  "Fred", "Gav", "Hal",  "Ian", "Jon",
	"Abi", "Bea", "Cath", "Dee", "Eve", "Fay",  "Gay", "Hope", "Ivy", "Jan"
};
int pref[jan + 1][jon + 1] = {
	{ abi, eve, cath, ivy, jan, dee, fay, bea, hope, gay },
	{ cath, hope, abi, dee, eve, fay, bea, jan, ivy, gay },
	{ hope, eve, abi, dee, bea, fay, ivy, gay, cath, jan },
	{ ivy, fay, dee, gay, hope, eve, jan, bea, cath, abi },
	{ jan, dee, bea, cath, fay, eve, abi, ivy, hope, gay },
	{ bea, abi, dee, gay, eve, ivy, cath, jan, hope, fay },
	{ gay, eve, ivy, bea, cath, abi, dee, hope, jan, fay },
	{ abi, eve, hope, fay, ivy, cath, jan, bea, gay, dee },
	{ hope, cath, dee, gay, bea, abi, fay, ivy, jan, eve },
	{ abi, fay, jan, gay, eve, bea, dee, cath, ivy, hope },

	{ bob, fred, jon, gav, ian, abe, dan, ed, col, hal   },
	{ bob, abe, col, fred, gav, dan, ian, ed, jon, hal   },
	{ fred, bob, ed, gav, hal, col, ian, abe, dan, jon   },
	{ fred, jon, col, abe, ian, hal, gav, dan, bob, ed   },
	{ jon, hal, fred, dan, abe, gav, col, ed, ian, bob   },
	{ bob, abe, ed, ian, jon, dan, fred, gav, col, hal   },
	{ jon, gav, hal, fred, bob, abe, col, ed, dan, ian   },
	{ gav, jon, bob, abe, ian, dan, hal, ed, col, fred   },
	{ ian, col, hal, gav, fred, bob, abe, ed, jon, dan   },
	{ ed, hal, gav, abe, bob, jon, col, ian, fred, dan   },
};
int pairs[jan + 1], proposed[jan + 1];

void engage(int man, int woman)
{
	pairs[man] = woman;
	pairs[woman] = man;
	if (verbose) printf("%4s is engaged to %4s\n", name[man], name[woman]);
}

void dump(int woman, int man)
{
	pairs[man] = pairs[woman] = clown;
	if (verbose) printf("%4s dumps %4s\n", name[woman], name[man]);
}


int rank(int this, int that)
{
	int i;
	for (i = abe; i <= jon && pref[this][i] != that; i++);
	return i;
}

void propose(int man, int woman)
{
	int fiance = pairs[woman];
	if (verbose) printf("%4s proposes to %4s\n", name[man], name[woman]);

	if (fiance == clown) {
		engage(man, woman);
	} else if (rank(woman, man) < rank(woman, fiance)) {
		dump(woman, fiance);
		engage(man, woman);
	}
}

int covet(int man1, int wife2)
{
	if (rank(man1, wife2) < rank(man1, pairs[man1]) &&
			rank(wife2, man1) < rank(wife2, pairs[wife2])) {
		printf( "    %4s (w/ %4s) and %4s (w/ %4s) prefer each other"
			" over current pairing.\n",
			name[man1], name[pairs[man1]], name[wife2], name[pairs[wife2]]
		);
		return 1;
	}
	return 0;
}

int thy_neighbors_wife(int man1, int man2)
{	
	return covet(man1, pairs[man2]) + covet(man2, pairs[man1]);
}

int unstable()
{
	int i, j, bad = 0;
	for (i = abe; i < jon; i++) {
		for (j = i + 1; j <= jon; j++)
			if (thy_neighbors_wife(i, j)) bad = 1;
	}
	return bad;
}

int main()
{
	int i, unengaged;
	
	for (i = abe; i <= jan; i++)
		pairs[i] = proposed[i] = clown;

	
	do {
		unengaged = 0;
		for (i = abe; i <= jon; i++) {
		
			if (pairs[i] != clown) continue;
			unengaged = 1;
			propose(i, pref[i][++proposed[i]]);
		}
	} while (unengaged);

	printf("Pairing:\n");
	for (i = abe; i <= jon; i++)
		printf("  %4s - %s\n", name[i],
			pairs[i] == clown ? "clown" : name[pairs[i]]);

	printf(unstable()
		? "Marriages not stable\n" 
		: "Stable matchup\n");

	printf("\nBut if Bob and Fred were to swap:\n");
	i = pairs[bob];
	engage(bob, pairs[fred]);
	engage(fred, i);
	printf(unstable() ? "Marriages not stable\n" : "Stable matchup\n");

	return 0;
}

Go: package main

import "fmt"







type proposers map[string][]string
    
var mPref = proposers{
    "abe": []string{
        "abi", "eve", "cath", "ivy", "jan",
        "dee", "fay", "bea", "hope", "gay"},
    "bob": []string{
        "cath", "hope", "abi", "dee", "eve",
        "fay", "bea", "jan", "ivy", "gay"},
    "col": []string{
        "hope", "eve", "abi", "dee", "bea",
        "fay", "ivy", "gay", "cath", "jan"},
    "dan": []string{
        "ivy", "fay", "dee", "gay", "hope",
        "eve", "jan", "bea", "cath", "abi"},
    "ed": []string{
        "jan", "dee", "bea", "cath", "fay",
        "eve", "abi", "ivy", "hope", "gay"},
    "fred": []string{
        "bea", "abi", "dee", "gay", "eve",
        "ivy", "cath", "jan", "hope", "fay"},
    "gav": []string{
        "gay", "eve", "ivy", "bea", "cath",
        "abi", "dee", "hope", "jan", "fay"},
    "hal": []string{
        "abi", "eve", "hope", "fay", "ivy",
        "cath", "jan", "bea", "gay", "dee"},
    "ian": []string{
        "hope", "cath", "dee", "gay", "bea",
        "abi", "fay", "ivy", "jan", "eve"},
    "jon": []string{
        "abi", "fay", "jan", "gay", "eve",
        "bea", "dee", "cath", "ivy", "hope"},
}

type recipients map[string]map[string]int

var wPref = recipients{
    "abi": map[string]int{
        "bob": 1, "fred": 2, "jon": 3, "gav": 4, "ian": 5,
        "abe": 6, "dan": 7, "ed": 8, "col": 9, "hal": 10},
    "bea": map[string]int{
        "bob": 1, "abe": 2, "col": 3, "fred": 4, "gav": 5,
        "dan": 6, "ian": 7, "ed": 8, "jon": 9, "hal": 10},
    "cath": map[string]int{
        "fred": 1, "bob": 2, "ed": 3, "gav": 4, "hal": 5,
        "col": 6, "ian": 7, "abe": 8, "dan": 9, "jon": 10},
    "dee": map[string]int{
        "fred": 1, "jon": 2, "col": 3, "abe": 4, "ian": 5,
        "hal": 6, "gav": 7, "dan": 8, "bob": 9, "ed": 10},
    "eve": map[string]int{
        "jon": 1, "hal": 2, "fred": 3, "dan": 4, "abe": 5,
        "gav": 6, "col": 7, "ed": 8, "ian": 9, "bob": 10},
    "fay": map[string]int{
        "bob": 1, "abe": 2, "ed": 3, "ian": 4, "jon": 5,
        "dan": 6, "fred": 7, "gav": 8, "col": 9, "hal": 10},
    "gay": map[string]int{
        "jon": 1, "gav": 2, "hal": 3, "fred": 4, "bob": 5,
        "abe": 6, "col": 7, "ed": 8, "dan": 9, "ian": 10},
    "hope": map[string]int{
        "gav": 1, "jon": 2, "bob": 3, "abe": 4, "ian": 5,
        "dan": 6, "hal": 7, "ed": 8, "col": 9, "fred": 10},
    "ivy": map[string]int{
        "ian": 1, "col": 2, "hal": 3, "gav": 4, "fred": 5,
        "bob": 6, "abe": 7, "ed": 8, "jon": 9, "dan": 10},
    "jan": map[string]int{
        "ed": 1, "hal": 2, "gav": 3, "abe": 4, "bob": 5,
        "jon": 6, "col": 7, "ian": 8, "fred": 9, "dan": 10},
}

func main() {
    
    ps := pair(mPref, wPref)
    
    fmt.Println("\nresult:")
    if !validateStable(ps, mPref, wPref) {
        return 
    }
    
    for {
        i := 0 
        var w2, m2 [2]string
        for w, m := range ps {
            w2[i] = w
            m2[i] = m
            if i == 1 {
                break
            }
            i++
        }
        fmt.Println("\nexchanging partners of", m2[0], "and", m2[1])
        ps[w2[0]] = m2[1]
        ps[w2[1]] = m2[0] 
        
        if !validateStable(ps, mPref, wPref) {
            return
        }
        
    }
}   

type parings map[string]string 
    

func pair(pPref proposers, rPref recipients) parings {
    
    pFree := proposers{}
    for k, v := range pPref {
        pFree[k] = append([]string{}, v...)
    }
    rFree := recipients{}
    for k, v := range rPref {
        rFree[k] = v
    }
    
    
    type save struct {
        proposer string
        pPref    []string
        rPref    map[string]int
    }
    proposals := map[string]save{} 

    
    
    for len(pFree) > 0 { 
        var proposer string
        var ppref []string
        for proposer, ppref = range pFree {
            break 
        }
        if len(ppref) == 0 {
            continue 
        }
        
        recipient := ppref[0] 
        ppref = ppref[1:]     
        var rpref map[string]int
        var ok bool
        
        if rpref, ok = rFree[recipient]; ok {
            
            
        } else {
            
            s := proposals[recipient] 
            
            if s.rPref[proposer] < s.rPref[s.proposer] {
                fmt.Println("engagement broken:", recipient, s.proposer)
                
                pFree[s.proposer] = s.pPref 
                
                rpref = s.rPref
                
            } else {
                
                pFree[proposer] = ppref 
                continue
            } 
        }
        fmt.Println("engagement:", recipient, proposer)
        proposals[recipient] = save{proposer, ppref, rpref}
        delete(pFree, proposer)
        delete(rFree, recipient)
    }
    
    ps := parings{}
    for recipient, s := range proposals {
        ps[recipient] = s.proposer
    }
    return ps
}

func validateStable(ps parings, pPref proposers, rPref recipients) bool {
    for r, p := range ps {
        fmt.Println(r, p)
    }
    for r, p := range ps {
        for _, rp := range pPref[p] {
            if rp == r {
                break
            }
            rprefs := rPref[rp]
            if rprefs[p] < rprefs[ps[rp]] {
                fmt.Println("unstable.")
                fmt.Printf("%s and %s would prefer each other over"+
                    " their current pairings.\n", p, rp)
                return false
            }
        }
    }
    fmt.Println("stable.")
    return true
}