Merge Sort in ALGOL 60

Welcome to the Merge Sort in ALGOL 60 page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

begin
    procedure usage;
    begin
        outstring(
            1,
            "Usage: please provide a list of at least two integers "
            "to sort in the format \"1, 2, 3, 4, 5\"\n"
        );
        stop
    end usage;

    comment Input a digit character from stdin and return the following:
        - "0" to "9" maps to 0 to 9
        - "+" maps to 10
        - "-" maps to 11
        - whitespace maps to 12
        - comma maps to 13
        - null byte maps to -1
        - invalid bytes map to -2;
    integer procedure indigit;
    begin
        comment Mapping:
            - "0" to "9" maps to 1 to 10
            - "+" maps to 11
            - "-" maps to 12
            - "\t" maps to 13
            - "\r" maps to 14
            - "\n" maps to 15
            - " " maps to 16
            - "," maps to 17
            - null byte maps to 18
            - invalid byte maps 0;
        integer ch;
        inchar(0, "0123456789+-\t\r\n ,", ch);
        if ch < 1 then ch := -2
        else if ch < 13 then ch := ch - 1
        else if ch < 17 then ch := 12
        else if ch = 17 then ch := 13
        else ch := -1;
        indigit := ch
    end indigit;

    comment Input an integer from stdin into 'result' and parse it.
        The last character is read into 'ch'.
        return true if integer is valid, false otherwise;
    boolean procedure inValidInteger(result, ch, allowComma);
    value allowComma;
    integer result, ch;
    boolean allowComma;
    begin
        boolean valid, commaFound;
        integer s;

        result := 0;
        valid := false;
        commaFound := false;
        s := 1;

        comment Ignore whitespace;
    whiteloop:
        ch := indigit;
        if ch = 12 then goto whiteloop;

        comment Process signs: ignore "+" and invert sign if "-";
    signloop:
        if ch = 10 | ch = 11 then
        begin
            if ch = 11 then s := -s;
            ch := indigit;
            goto signloop
        end;

        comment Indicate valid if "0" to "9";
        if ch >= 0 & ch <= 9 then valid := true;

        comment Process digits: update value;
    valueloop:
        if ch >= 0 & ch <= 9 then
        begin
            comment Invalid if overflow or underflow;
            if (s > 0 & (maxint - ch) % 10 < result) |
                (s < 0 & (-1 - maxint + ch) % 10 > result) then valid := false;
            
            result := result * 10 + s * ch;
            ch := indigit;
            goto valueloop
        end;

        comment If comma not allowed, ignore characters until end
            input. If comma allowed, ignore characters until comma
            or end of input. Indicate if comma found;
    ignoreloop:
        if !(ch = -1 | (allowComma & ch = 13)) then
        begin
            if ch != 12 & ch != 13 then valid := false;
            if ch = 13 then
            begin
                commaFound := true;
                if !allowComma then valid := false
            end;

            ch := indigit;
            goto ignoreloop
        end;

        comment If comma found, indicate last character is comma;
        if commaFound then ch := 13;

        inValidInteger := valid
    end inValidInteger;

    comment Returns length of array if valid, -1 otherwise;
    integer procedure inIntegerArray(arr, maxLen);
    value maxLen;
    integer array arr;
    integer maxLen;
    begin
        integer arrLen, val, ch;
        boolean valid;

        arrLen := 0;

        comment Get value with possible comma (13). Indicate invalid,
            if invalid integer. Otherwise, append value to array if
            no invalid values and there is room;
    itemloop:
        if !inValidInteger(val, ch, true) then arrLen := -1
        else if arrLen >= 0 & arrLen < maxLen then
        begin
            arrLen := arrLen + 1;
            arr[arrLen] := val
        end;

        comment Repeat until end of input;
        if ch != -1 then goto itemloop;

        inIntegerArray := arrLen
    end inIntegerArray;

    comment Output integer without space. This is needed since ALGOL60
        'outinteger' automatically adds a space after the integer.
        Source: 'outinteger' function source code in Appendix 2 of
        https://www.algol60.org/reports/algol60_mr.pdf;
    procedure outIntegerNoSpace(x);
    value x;
    integer x;
    begin
        procedure digits(x);
        value x;
        integer x;
        begin
            integer d;
            d := x % 10;
            x := x - 10 * d;
            if d != 0 then digits(d);
            outchar(1, "0123456789", iabs(x) + 1)
        end digits;
        if x < 0 then outstring(1, "-");
        digits(x)
    end outIntegerNoSpace;

    comment Output integer array;
    procedure outIntegerArray(arr, arrLen);
    value arrLen;
    integer array arr;
    integer arrLen;
    begin
        integer i;
        for i := 1 step 1 until arrLen do
        begin
            if i > 1 then outstring(1, ", ");
            outIntegerNoSpace(arr[i])
        end;

        if arrLen > 0 then outstring(1, "\n")
    end outIntegerArray;

    comment Source https://en.wikipedia.org/wiki/Merge_sort#Top-down_implementation;
    procedure mergeSort(arrA, n);
    value n;
    integer array arrA;
    integer n;
    begin
        integer i;
        integer array arrB[1:n];

        comment Copy array A into array B;
        for i := 1 step 1 until n do arrB[i] := arrA[i];

        comment Run merge sort recursively;
        mergeSortRec(arrA, 1, n + 1, arrB)
    end mergeSort;

    procedure mergeSortRec(arrA, lo, hi, arrB);
    value lo, hi;
    integer array arrA, arrB;
    integer lo, hi;
    begin
        integer mid;

        comment If more than 1 point;
        if hi - lo > 1 then
        begin
            comment Find the midpoint to split the array;
            mid := (lo + hi) % 2;

            comment Recursively sort the left and right halves into array B;
            mergeSortRec(arrB, lo, mid, arrA);
            mergeSortRec(arrB, mid, hi, arrA);

            comment Merge sorted halves back into array A;
            merge(arrB, lo, mid, hi, arrA)
        end
    end mergeSortRec;

    procedure merge(arrA, lo, mid, hi, arrB);
    value lo, mid, hi;
    integer array arrA, arrB;
    integer lo, mid, hi;
    begin
        integer i, j, k;
        boolean useLeft;

        i := lo;
        j := mid;

        comment Merge the two sorted runs into array B;
        for k := lo step 1 until hi - 1 do
        begin
            comment Determine whether to use left half;
            useLeft := false;
            if i < mid then
            begin
                if j >= hi then useLeft := true
                else if arrA[i] <= arrA[j] then useLeft := true
            end;

            comment Take element from appropriate half;
            if useLeft then
            begin
                arrB[k] := arrA[i];
                i := i + 1
            end
            else
            begin
                arrB[k] := arrA[j];
                j := j + 1
            end
        end
    end merge;

    integer argc, arrLen;
    integer array arr[1:100];
    
    comment Get number of parameters. Exit if too few;
    ininteger(0, argc);
    if argc < 1 then usage;

    comment Get input from 1st argument. Exit if invalid;
    arrLen := inIntegerArray(arr, 100);
    if arrLen < 2 then usage;

    comment Merge sort input and output result;
    mergeSort(arr, arrLen);
    outIntegerArray(arr, arrLen)
end

Merge Sort in ALGOL 60 was written by:

• rzuckerm

If you see anything you'd like to change or update, please consider contributing.

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