diff --git a/src/creep/builder.ts b/src/creep/builder.ts
index 74498ca..3de1f89 100644
--- a/src/creep/builder.ts
+++ b/src/creep/builder.ts
@@ -1,8 +1,8 @@
-import { randomIntInRange } from '../utils/Random';
+import { selectSource } from './common';
 
 interface BuilderMemory extends CreepMemory {
   building: boolean | undefined;
-  sourceNum: number | undefined;
+  sourceId: Id<Source> | undefined;
 }
 
 export function runAsBuilder(creep: Creep) {
@@ -16,9 +16,8 @@ export function runAsBuilder(creep: Creep) {
     memory.building = true;
   }
 
-  if (memory.sourceNum === undefined) {
-    const sources = creep.room.find(FIND_SOURCES);
-    memory.sourceNum = randomIntInRange(0, sources.length);
+  if (memory.sourceId === undefined) {
+    memory.sourceId = selectSource(creep);
   }
 
   if (memory.building) {
@@ -34,11 +33,10 @@ export function runAsBuilder(creep: Creep) {
       creep.moveTo(spawns[0]);
     }
   } else {
-    const sources = creep.room.find(FIND_SOURCES);
-    const sourceNum = memory.sourceNum;
-    if (creep.harvest(sources[sourceNum]) == ERR_NOT_IN_RANGE) {
-      creep.say('🔄 harvest ' + sourceNum);
-      creep.moveTo(sources[sourceNum], { visualizePathStyle: { stroke: '#ffaa00' } });
+    const source = Game.getObjectById(memory.sourceId);
+    if (source && creep.harvest(source) == ERR_NOT_IN_RANGE) {
+      creep.say('🔄 harvest');
+      creep.moveTo(source, { visualizePathStyle: { stroke: '#ffaa00' } });
     }
   }
 }
diff --git a/src/creep/common.ts b/src/creep/common.ts
index 50d01fc..71764ed 100644
--- a/src/creep/common.ts
+++ b/src/creep/common.ts
@@ -1,22 +1,21 @@
-import { randomIntInRange } from '../utils/Random';
+import { randomValueFromArray } from '../utils/Random';
 
-export interface SourceNumber {
-  sourceNum: number | undefined;
-}
-
-function selectSource(memory: SourceNumber) {
-  if (memory.sourceNum === undefined) {
-    const sources = creep.room.find(FIND_SOURCES);
-    memory.sourceNum = randomIntInRange(0, sources.length);
-  }
-}
-
-function moveToSource(creep: Creep) {
-  const memory = creep.memory as SourceNumber;
+export function selectSource(creep: Creep): Id<Source> {
   const sources = creep.room.find(FIND_SOURCES);
-  const sourceNum = memory.sourceNum;
-  if (creep.harvest(sources[sourceNum]) == ERR_NOT_IN_RANGE) {
-    creep.say('🔄 harvest ' + sourceNum);
-    creep.moveTo(sources[sourceNum], { visualizePathStyle: { stroke: '#ffaa00' } });
+  const choices: Array<String> = [];
+  for (let source of sources) {
+    const weight = getSourceWeight(source);
+    for (let i = 0; i < weight; ++i) {
+      choices.push(source.id);
+    }
   }
+  console.log('Sources', choices);
+  return randomValueFromArray(choices) as Id<Source>;
+}
+
+function getSourceWeight(source: Source) {
+  const pos = source.pos;
+  const terrains = source.room.lookForAtArea(LOOK_TERRAIN, pos.y - 1, pos.x - 1, pos.y + 1, pos.x + 1, true);
+  const walked = terrains.filter((t) => t.terrain !== 'wall');
+  return Math.max(walked.length - 1, 1);
 }
diff --git a/src/creep/harvester.ts b/src/creep/harvester.ts
index 2e70ce5..f3bd67c 100644
--- a/src/creep/harvester.ts
+++ b/src/creep/harvester.ts
@@ -1,23 +1,21 @@
-import { randomIntInRange } from '../utils/Random';
+import { selectSource } from './common';
 
 interface HarvesterMemory extends CreepMemory {
-  sourceNum: number | undefined;
+  sourceId: Id<Source> | undefined;
 }
 
 export function runAsHarvester(creep: Creep) {
   const memory = creep.memory as HarvesterMemory;
 
-  if (memory.sourceNum === undefined) {
-    const sources = creep.room.find(FIND_SOURCES);
-    memory.sourceNum = randomIntInRange(0, sources.length);
+  if (memory.sourceId === undefined) {
+    memory.sourceId = selectSource(creep);
   }
 
   if (creep.store.getFreeCapacity() > 0) {
-    let sources = creep.room.find(FIND_SOURCES);
-    const sourceNum = memory.sourceNum;
-    if (creep.harvest(sources[sourceNum]) === ERR_NOT_IN_RANGE) {
-      creep.say('🔄 harvest ' + sourceNum);
-      creep.moveTo(sources[sourceNum], { visualizePathStyle: { stroke: '#ffaa00' } });
+    const source = Game.getObjectById(memory.sourceId);
+    if (source && creep.harvest(source) === ERR_NOT_IN_RANGE) {
+      creep.say('🔄 harvest');
+      creep.moveTo(source, { visualizePathStyle: { stroke: '#ffaa00' } });
     }
   } else {
     let targets = creep.room.find(FIND_STRUCTURES, {
diff --git a/src/creep/upgrader.ts b/src/creep/upgrader.ts
index 80eff48..034f7c0 100644
--- a/src/creep/upgrader.ts
+++ b/src/creep/upgrader.ts
@@ -1,8 +1,8 @@
-import { randomIntInRange } from '../utils/Random';
+import { selectSource } from './common';
 
 interface UpgraderMemory extends CreepMemory {
   upgrading: boolean | undefined;
-  sourceNum: number | undefined;
+  sourceId: Id<Source> | undefined;
 }
 
 export function runAsUpgrader(creep: Creep) {
@@ -16,9 +16,8 @@ export function runAsUpgrader(creep: Creep) {
     memory.upgrading = true;
   }
 
-  if (memory.sourceNum === undefined) {
-    const sources = creep.room.find(FIND_SOURCES);
-    memory.sourceNum = randomIntInRange(0, sources.length);
+  if (memory.sourceId === undefined) {
+    memory.sourceId = selectSource(creep);
   }
 
   if (memory.upgrading) {
@@ -27,11 +26,10 @@ export function runAsUpgrader(creep: Creep) {
       creep.moveTo(creep.room.controller, { visualizePathStyle: { stroke: '#ffffff' } });
     }
   } else {
-    const sources = creep.room.find(FIND_SOURCES);
-    const sourceNum = memory.sourceNum;
-    if (creep.harvest(sources[sourceNum]) == ERR_NOT_IN_RANGE) {
-      creep.say('🔄 harvest ' + sourceNum);
-      creep.moveTo(sources[sourceNum], { visualizePathStyle: { stroke: '#ffaa00' } });
+    const source = Game.getObjectById(memory.sourceId);
+    if (source && creep.harvest(source) == ERR_NOT_IN_RANGE) {
+      creep.say('🔄 harvest');
+      creep.moveTo(source, { visualizePathStyle: { stroke: '#ffaa00' } });
     }
   }
 }
diff --git a/src/main.ts b/src/main.ts
index a86a42c..f8a6879 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -4,8 +4,8 @@ import { runAsBuilder } from './creep/builder';
 import { runAsUpgrader } from './creep/upgrader';
 import { uniqId } from './utils/Identity';
 
-function makeName(role: string): string {
-  return uniqId(role + '_');
+function makeName(): string {
+  return uniqId();
 }
 
 enum CreepRole {
@@ -19,17 +19,35 @@ function makeCreep(role: CreepRole, count: number) {
   console.log(`Make creep "${role}"`, 'need', count, 'has', creeps.length);
   if (creeps.length < count) {
     const firstSpawn = _.first(Object.values(Game.spawns));
-    const name = makeName(role);
+    const name = makeName();
     const memory = { role: role } as CreepMemory;
     const err = firstSpawn.spawnCreep([WORK, CARRY, MOVE], name, { memory });
     console.log(`Make creep "${role}"`, 'err', err);
   }
 }
 
+function callHarvestersFromOthers(minHarvCount: number) {
+  const harvesters = Object.values(Game.creeps).filter((c) => c.memory.role === CreepRole.HARVESTER);
+  if (harvesters.length < minHarvCount) {
+    const others = Object.values(Game.creeps).filter((c) => c.memory.role !== CreepRole.HARVESTER);
+    const required = Math.min(minHarvCount, others.length);
+    console.log('Call harvesters', required);
+    let count = 0;
+    for (let creep of others) {
+      if (count >= required) {
+        break;
+      }
+      count++;
+      creep.memory.role = CreepRole.HARVESTER;
+    }
+  }
+}
+
 // When compiling TS to JS and bundling with rollup, the line numbers and file names in error messages change
 // This utility uses source maps to get the line numbers and file names of the original, TS source code
 export const loop = ErrorMapper.wrapLoop(() => {
-  console.log(`\nCurrent game tick is ${Game.time}`);
+  console.log('');
+  console.log(`Current game tick is ${Game.time}`);
 
   // Automatically delete memory of missing creeps
   for (const name in Memory.creeps) {
@@ -38,10 +56,15 @@ export const loop = ErrorMapper.wrapLoop(() => {
     }
   }
 
-  makeCreep(CreepRole.HARVESTER, 6);
+  const harvCount = 8;
+  const minHarvCount = Math.floor(harvCount * 0.6);
+
+  makeCreep(CreepRole.HARVESTER, harvCount);
   makeCreep(CreepRole.UPGRADER, 6);
   makeCreep(CreepRole.BUILDER, 6);
 
+  callHarvestersFromOthers(minHarvCount);
+
   // Process current creeps
   for (let name in Game.creeps) {
     const creep = Game.creeps[name];
diff --git a/src/utils/Random.ts b/src/utils/Random.ts
index 9a4ddca..ecd379f 100644
--- a/src/utils/Random.ts
+++ b/src/utils/Random.ts
@@ -2,3 +2,9 @@ export function randomIntInRange(min: number, max: number): number {
   const delta = max - min;
   return Math.floor(Math.random() * delta) + min;
 }
+
+export function randomValueFromArray<T>(items: Array<T>): T {
+  const len = items.length;
+  const index = randomIntInRange(0, len);
+  return items[index];
+}