[marketplace] Specify size per slot, instead of total size

Reasoning: it was unclear exactly how much storage a
host should have available for a slot. The division
size/slots can have a non-integer value.

contracts/Marketplace.sol

@@ -129,11 +129,11 @@ contract Marketplace is Collateral, Proofs {
   }
 
   struct Ask {
-    uint256 size; // size of requested storage in number of bytes
+    uint64 slots; // the number of requested slots
+    uint256 slotSize; // amount of storage per slot (in number of bytes)
     uint256 duration; // how long content should be stored (in seconds)
     uint256 proofProbability; // how often storage proofs are required
     uint256 reward; // amount of tokens paid per second per slot to hosts
-    uint64 slots; // the total number of hosts that store the data set
   }
 
   struct Content {

test/examples.js

@@ -5,11 +5,11 @@ const { hexlify, randomBytes } = ethers.utils
 const exampleRequest = () => ({
   client: hexlify(randomBytes(20)),
   ask: {
-    size: 1 * 1024 * 1024 * 1024, // 1 Gigabyte
+    slots: 4,
+    slotSize: 1 * 1024 * 1024 * 1024, // 1 Gigabyte
     duration: hours(10),
     proofProbability: 4, // require a proof roughly once every 4 periods
     reward: 84,
-    slots: 4,
   },
   content: {
     cid: "zb2rhheVmk3bLks5MgzTqyznLu1zqGH5jrfTA1eAZXrjx7Vob",

test/ids.js

@@ -2,7 +2,7 @@ const { ethers } = require("hardhat")
 const { keccak256, defaultAbiCoder } = ethers.utils
 
 function requestId(request) {
-  const Ask = "tuple(uint256, uint256, uint256, uint256, int64)"
+  const Ask = "tuple(int64, uint256, uint256, uint256, uint256)"
   const Erasure = "tuple(uint64)"
   const PoR = "tuple(bytes, bytes, bytes)"
   const Content = "tuple(string, " + Erasure + ", " + PoR + ")"
@@ -12,7 +12,13 @@ function requestId(request) {
 }
 
 function askToArray(ask) {
-  return [ask.size, ask.duration, ask.proofProbability, ask.reward, ask.slots]
+  return [
+    ask.slots,
+    ask.slotSize,
+    ask.duration,
+    ask.proofProbability,
+    ask.reward,
+  ]
 }
 
 function erasureToArray(erasure) {