Task: Abbreviations, automatic
C++: #include <iomanip>
#include <iostream>
#include <fstream>
#include <map>
#include <sstream>
#include <string>
#include <vector>

std::vector<std::string> split(const std::string& str, char delimiter) {
    std::vector<std::string> tokens;
    std::string token;
    std::istringstream tokenStream(str);
    while (std::getline(tokenStream, token, delimiter)) {
        tokens.push_back(token);
    }
    return tokens;
}

int main() {
    using namespace std;
    string line;
    int i = 0;

    ifstream in("days_of_week.txt");
    if (in.is_open()) {
        while (getline(in, line)) {
            i++;
            if (line.empty()) {
                continue;
            }

            auto days = split(line, ' ');
            if (days.size() != 7) {
                throw std::runtime_error("There aren't 7 days in line " + i);
            }

            map<string, int> temp;
            for (auto& day : days) {
                if (temp.find(day) != temp.end()) {
                    cerr << " ∞  " << line << '\n';
                    continue;
                }
                temp[day] = 1;
            }

            int len = 1;
            while (true) {
                temp.clear();
                for (auto& day : days) {
                    string key = day.substr(0, len);
                    if (temp.find(key) != temp.end()) {
                        break;
                    }
                    temp[key] = 1;
                }
                if (temp.size() == 7) {
                    cout << setw(2) << len << "  " << line << '\n';
                    break;
                }
                len++;
            }
        }
    }

    return 0;
}

Go: package main

import(
    "bufio"
    "fmt"
    "os"
    "strings"
)

func distinctStrings(strs []string) []string {
    len := len(strs)
    set := make(map[string]bool, len)
    distinct := make([]string, 0, len)
    for _, str := range strs {
        if !set[str] {
            distinct = append(distinct, str)
            set[str] = true
        }
    }
    return distinct
}

func takeRunes(s string, n int) string {
    i := 0
    for j := range s {
        if i == n {
            return s[:j]
        }
        i++
    }
    return s
}

func main() {
    file, err := os.Open("days_of_week.txt")
    if err != nil {
        fmt.Println("Unable to open file.")
        return
    }
    defer file.Close()
    reader := bufio.NewReader(file)
    lineCount := 0
    for {
        line, err := reader.ReadString('\n')
        if err != nil { 
            return
        }
        line = strings.TrimSpace(line)
        lineCount++
        if line == "" {
            fmt.Println()
            continue
        }
        days := strings.Fields(line)
        daysLen := len(days)
        if (len(days) != 7) {
            fmt.Println("There aren't 7 days in line", lineCount)
            return
        }
        if len(distinctStrings(days)) != 7 { 
            fmt.Println(" ∞ ", line)
            continue
        }
        for abbrevLen := 1; ; abbrevLen++ {
            abbrevs := make([]string, daysLen)
            for i := 0; i < daysLen; i++ {
                abbrevs[i] = takeRunes(days[i], abbrevLen)
            }
            if len(distinctStrings(abbrevs)) == 7 {
                fmt.Printf("%2d  %s\n", abbrevLen, line)
                break
            }
        }
    }
}