DEX trade tutorial: more progress (incomplete)

There's still a bug that causes the code's predictions to be wrong.

content/_code-samples/trade-in-the-decentralized-exchange/js/trade-in-the-dex.js

@@ -15,17 +15,113 @@ async function main() {
   await client.connect()
 
   // Get credentials from the Testnet Faucet -----------------------------------
-  console.log("Requesting address from the Testnet faucet...")
-  const wallet = (await client.fundWallet()).wallet
-  console.log(`Got address ${wallet.address}.`)
+  // console.log("Requesting address from the Testnet faucet...")
+  // const wallet = (await client.fundWallet()).wallet
+  // console.log(`Got address ${wallet.address}.`)
+  const wallet = xrpl.Wallet.fromSeed("SEED VALUE HERE") // temp for testing: don't fund every time
+
+  // Define the proposed trade.
+  const we_want = {
+    currency: "TST",
+    issuer: "rP9jPyP5kyvFRb6ZiRghAGw5u8SGAmU4bd",
+    value: "25"}
+
+                              // 250 TST * 10 XRP per TST * 1.15 fx cost
+  const we_spend = {currency: "XRP", value: xrpl.xrpToDrops(25*10*1.15)}
+  // "Quality" is defined as TakerPays ÷ TakerGets. The lower the "quality"
+  // number, the better the proposed exchange rate is for the taker.
+  const proposed_quality = Number(we_spend.value / we_want.value)
+
+  // Look up Offers. -----------------------------------------------------------
+  // To buy TST, look up Offers where "TakerGets" is TST:
+  const orderbook_resp = await client.request({
+    "command": "book_offers",
+    "taker": wallet.address,
+    "ledger_index": "current",
+    "taker_gets": we_want,
+    "taker_pays": we_spend
+  })
+  console.log(JSON.stringify(orderbook_resp.result, null, 2))
+
+  // Estimate whether a proposed Offer would execute immediately, and...
+  // If so, how much of it? (Partial execution is possible)
+  // If not, how much liquidity is above it? (How deep in the order book would
+  //    other Offers have to go before ours would get taken?)
+
+  const offers = orderbook_resp.result.offers
+  const want_amt = Number(we_want.value)
+  let running_total = 0
+  if (!offers) {
+    console.log(`No Offers in the matching book.
+                 Offer probably won't execute immediately.`)
+  } else {
+    for (o of offers) {
+      if (o.quality <= proposed_quality) {
+        console.log(`Matching Offer found, funded with ${o.owner_funds}`)
+        running_total += Number(o.owner_funds)
+        if (running_total >= want_amt) {
+          console.log("Full Offer will probably fill")
+          break
+        }
+      } else {
+        // Offers are in ascending quality order, so no others after this
+        // will match, either
+        console.log(`Remaining orders too expensive.`)
+        break
+      }
+    }
+    console.log(`Total matched: ${Math.min(running_total, want_amt)}`)
+    if (running_total > 0 && running_total < want_amt) {
+      console.log(`Remaining ${want_amt - running_total} would probably be
+                   placed on top of the order book.`)
+    }
+  }
+
+  if (running_total = 0) {
+    // If part of the Offer was expected to cross, then the rest would be placed
+    // at the top of the order book. If none did, then there might be other
+    // Offers going the same direction as ours already on the books with an
+    // equal or better rate. This code counts how much liquidity is likely to be
+    // above ours.
+
+    // Unlike above, this time we check for Offers going the same direction as
+    // ours, so TakerGets and TakerPays are reversed from the previous
+    // book_offers request.
+    const orderbook2_resp = await client.request({
+      "command": "book_offers",
+      "taker": wallet.address,
+      "ledger_index": "current",
+      "taker_gets": we_spend,
+      "taker_pays": we_want
+    })
+    console.log(JSON.stringify(orderbook2_resp.result, null, 2))
+
+    // Since TakerGets/TakerPays are reversed, the quality is the inverse.
+    // You could also calculate this as 1/proposed_quality.
+    const offered_quality = Number(we_want.value / we_spend.value)
+
+    const offers2 = orderbook2_resp.result.offers
+    let running_total2 = 0
+    if (!offers2) {
+      console.log(`No similar Offers in the book. Ours would be the first.`)
+    } else {
+      for (o of offers2) {
+        if (o.quality <= offered_quality) {
+          console.log(`Existing offer found, funded with ${o.owner_funds}`)
+          running_total2 += Number(o.owner_funds)
+        } else {
+          console.log(`Remaining orders are below where ours would be placed.`)
+          break
+        }
+      }
+      console.log(`Our Offer would be placed below at least ${running_total2} ${we_spend.currency}`)
+      if (running_total > 0 && running_total < want_amt) {
+        console.log(`Remaining ${want_amt - running_total} will probably be
+                     placed on top of the order book.`)
+      }
+    }
+  }
 
-  // Look up Offers
-  const orderbook = await client.getOrderbook(
-      {currency: "TST", issuer: "rP9jPyP5kyvFRb6ZiRghAGw5u8SGAmU4bd"}, // TakerPays
-      {currency: "XRP"}, // TakerGets
-      {ledger_index: "current", taker: wallet.address} // options
-  )
-  console.log(JSON.stringify(orderbook, null, 2))
 
   // Depending on your use case, you may want to present options to the user
   // here with estimates of the cost to buy (or proceeds from selling) the
@@ -37,12 +133,8 @@ async function main() {
   const offer_1 = {
     "TransactionType": "OfferCreate",
     "Account": wallet.address,
-    "TakerPays": {
-      "currency": "TST",
-      "issuer": "rP9jPyP5kyvFRb6ZiRghAGw5u8SGAmU4bd",
-      "value": "25"
-    },
-    "TakerGets": xrpl.xrpToDrops(300)
+    "TakerPays": we_want,
+    "TakerGets": we_spend.value // since it's XRP
   }
 
   const prepared = await client.autofill(offer_1)