javascript - Finding the middle index between any two indexes in a circular array -

i have array , 3 indexes:

var arr = ['a', 'b', 'c', 'd']; var f = 0; // first element var l = arr.length - 1; // last element var c = 0; // current element 

i'm trying write function makes index c cycle through array. requirement never reach index l. so, whenever c reaches value, f , l need increased well.

a reasonable value work limit c thought middle point between f , l. function wrote this:

var mod = function(x, m) {     var r = x%m;     return r<0 ? r+m : r; }  while (true) {     console.log('f', f, 'l', l, 'c', c);     if (c == l) {        console.log('error');     }      c = mod(c + 1, arr.length);     if (c > mod(l - f, arr.length) / 2) {       f = mod(f + 1, arr.length);       l = mod(l + 1, arr.length);    } } 

it doesn't work, clearly. there nice simple formula modulo operator want or approach totally wrong?

here's fiddle try it: https://jsfiddle.net/wku37h9e/2/

edit: explaining purpose of bit long. let's in array stored positions of external elements have move around. when c close l, advance indexes. graphically this:

1. [f|c] [   ] [   ] [ l ] 2. [ f ] [ c ] [   ] [ l ] 3. [ l ] [ f ] [ c ] [   ] // here start moving things around 4. [   ] [ l ] [ f ] [ c ] 5. [ c ] [   ] [ l ] [ f ] 6. [ f ] [ c ] [   ] [ l ] 

and on. more elements in array there more distance between indexes.

i use different normal version of mod take in account negative numbers.

i hope it's bit clearer now

a reasonable value work limit c thought middle point between f , l

finding mean average between 2 values, low , high, in mod len

function midpoint(low, high, len) {     var mid;     low %= len;     high %= len;     while (low < 0) low += len; // these 2 lines     while (high < low) high += len; // important direction     mid = (low + high) / 2; // mean average     mid %= len; // convert our mod     return mid; } 

so have

// works expected beyond range of mod midpoint( 1, -1, 9); // middle of 1..8 (mod 9) = 4.5 midpoint(-7,  5, 9); // middle of 2..5 (mod 9) = 3.5 midpoint(-6, 12, 9); // middle of 3..3 (mod 9) = 3 // , midpoint( 2,  5, 9); // middle of 2..5 (mod 9) = 3.5 midpoint( 5,  2, 9); // middle of 5..2 (mod 9) = 8 (we went other way around) 

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you don't have force mod until after whole calculation, if wanted following, lose direction around loop

function midpoint(a, b, len) {     var mid;     mid = (a + b) / 2; // mean average     mid %= len; // convert our mod     while (mid < 0) mid += len; // positive     return mid; } 

notice here, however

midpoint(-6, 3, 9); // middle of 3..3 (mod 9) = 7.5 midpoint( 3, 3 + 0 * 9, 9); // middle of 3..3 (mod 9) = 3 midpoint( 3, 3 + 1 * 9, 9); // middle of 3..3 (mod 9) = 7.5 midpoint( 3, 3 + 2 * 9, 9); // middle of 3..3 (mod 9) = 3 

i.e. mid point of 3..3 (mod 9) both 3 , 7.5, true, way around we've gone depends on k (where b - = k * len + a)