Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
Running Median. Finds the median of the last K inputs in O(lg K). See for some details.
//Copyright (c) 2011 under <>
#include <stdlib.h>
//Customize for your data Item type
typedef int Item;
#define ItemLess(a,b) ((a)<(b))
#define ItemMean(a,b) (((a)+(b))/2)
typedef struct Mediator_t
Item* data; //circular queue of values
int* pos; //index into `heap` for each value
int* heap; //max/median/min heap holding indexes into `data`.
int N; //allocated size.
int idx; //position in circular queue
int ct; //count of items in queue
} Mediator;
/*--- Helper Functions ---*/
#define minCt(m) (((m)->ct-1)/2) //count of items in minheap
#define maxCt(m) (((m)->ct)/2) //count of items in maxheap
//returns 1 if heap[i] < heap[j]
int mmless(Mediator* m, int i, int j)
return ItemLess(m->data[m->heap[i]],m->data[m->heap[j]]);
//swaps items i&j in heap, maintains indexes
int mmexchange(Mediator* m, int i, int j)
int t = m->heap[i];
return 1;
//swaps items i&j if i<j; returns true if swapped
int mmCmpExch(Mediator* m, int i, int j)
return (mmless(m,i,j) && mmexchange(m,i,j));
//maintains minheap property for all items below i/2.
void minSortDown(Mediator* m, int i)
for (; i <= minCt(m); i*=2)
{ if (i>1 && i < minCt(m) && mmless(m, i+1, i)) { ++i; }
if (!mmCmpExch(m,i,i/2)) { break; }
//maintains maxheap property for all items below i/2. (negative indexes)
void maxSortDown(Mediator* m, int i)
for (; i >= -maxCt(m); i*=2)
{ if (i<-1 && i > -maxCt(m) && mmless(m, i, i-1)) { --i; }
if (!mmCmpExch(m,i/2,i)) { break; }
//maintains minheap property for all items above i, including median
//returns true if median changed
int minSortUp(Mediator* m, int i)
while (i>0 && mmCmpExch(m,i,i/2)) i/=2;
return (i==0);
//maintains maxheap property for all items above i, including median
//returns true if median changed
int maxSortUp(Mediator* m, int i)
while (i<0 && mmCmpExch(m,i/2,i)) i/=2;
return (i==0);
/*--- Public Interface ---*/
//creates new Mediator: to calculate `nItems` running median.
//mallocs single block of memory, caller must free.
Mediator* MediatorNew(int nItems)
int size = sizeof(Mediator)+nItems*(sizeof(Item)+sizeof(int)*2);
Mediator* m= malloc(size);
m->data= (Item*)(m+1);
m->pos = (int*) (m->data+nItems);
m->heap = m->pos+nItems + (nItems/2); //points to middle of storage.
m->ct = m->idx = 0;
while (nItems--) //set up initial heap fill pattern: median,max,min,max,...
{ m->pos[nItems]= ((nItems+1)/2) * ((nItems&1)?-1:1);
return m;
//Inserts item, maintains median in O(lg nItems)
void MediatorInsert(Mediator* m, Item v)
int isNew=(m->ct<m->N);
int p = m->pos[m->idx];
Item old = m->data[m->idx];
m->idx = (m->idx+1) % m->N;
if (p>0) //new item is in minHeap
{ if (!isNew && ItemLess(old,v)) { minSortDown(m,p*2); }
else if (minSortUp(m,p)) { maxSortDown(m,-1); }
else if (p<0) //new item is in maxheap
{ if (!isNew && ItemLess(v,old)) { maxSortDown(m,p*2); }
else if (maxSortUp(m,p)) { minSortDown(m, 1); }
else //new item is at median
{ if (maxCt(m)) { maxSortDown(m,-1); }
if (minCt(m)) { minSortDown(m, 1); }
//returns median item (or average of 2 when item count is even)
Item MediatorMedian(Mediator* m)
Item v= m->data[m->heap[0]];
if ((m->ct&1)==0) { v= ItemMean(v,m->data[m->heap[-1]]); }
return v;
/*--- Test Code ---*/
#include <stdio.h>
void PrintMaxHeap(Mediator* m)
int i;
printf("Max: %3d",m->data[m->heap[-1]]);
for (i=2;i<=maxCt(m);++i)
printf("|%3d ",m->data[m->heap[-i]]);
if(++i<=maxCt(m)) printf("%3d",m->data[m->heap[-i]]);
void PrintMinHeap(Mediator* m)
int i;
printf("Min: %3d",m->data[m->heap[1]]);
for (i=2;i<=minCt(m);++i)
printf("|%3d ",m->data[m->heap[i]]);
if(++i<=minCt(m)) printf("%3d",m->data[m->heap[i]]);
void ShowTree(Mediator* m)
printf("Mid: %3d\n",m->data[m->heap[0]]);
int main(int argc, char* argv[])
int i,v;
Mediator* m = MediatorNew(15);
for (i=0;i<30;i++)
v = rand()&127;
printf("Inserting %3d \n",v);
printf("Median = %3d.\n\n",v);
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment