diff --git a/digits-work-sample-go.d/analyzer.go b/digits-work-sample-go.d/analyzer.go
index d71481d..b0e3d75 100644
--- a/digits-work-sample-go.d/analyzer.go
+++ b/digits-work-sample-go.d/analyzer.go
@@ -54,8 +54,13 @@ func (anz *Analyzer) LargestTransaction() (Transaction, error) {
 // ByCategory groups the transactions by category and returns a map of
 // category name to the list of transactions that are part of that category.
 func (anz *Analyzer) ByCategory() map[string]Transactions {
-	// TODO: Not implemented.
-	return nil
+	result := map[string]Transactions{}
+	for i := range anz.transactions {
+		transactionsInCategory := result[anz.transactions[i].Category]
+		transactionsInCategory = append(transactionsInCategory, anz.transactions[i])
+		result[anz.transactions[i].Category] = transactionsInCategory
+	}
+	return result
 }
 
 // BigSpenderReport generates our Big Spender Report.
diff --git a/todo.yaml b/todo.yaml
index c6e1e47..78f2c42 100755
--- a/todo.yaml
+++ b/todo.yaml
@@ -2,7 +2,6 @@ todo:
 - review readme tail to finish // answer 3 questions, gofmt
 - todo: go test
   subtasks:
-  - TestAnalyzer_GroupByCategory
   - TestAnalyzer_BigSpendersReport
   - TestAnalyzer_TransactionsFromURLs
   - TestAnalyzer_TransactionsFromURLsConcurrent
@@ -33,6 +32,9 @@ todo:
     can be pending and then later disappear to have their date updated OR be like
     pre-charges on credit cards that later disappear
   - analyzer.go:Analyzer:Add is not concurrency-safe
+  - analyzer.go:Analyzer:ByCategory probably allocates big slices out the gate and
+    expends to way-too-big-slices by end time. Could do 2 passes (still O(n)) to pre-compute
+    each category's size. Makes code look weird, though. Hm.
 scheduled: []
 done:
 - todo: hello world
@@ -602,3 +604,74 @@ done:
     pre-charges on credit cards that later disappear
   - analyzer.go:Analyzer:Add is not concurrency-safe
   ts: Sun Oct 15 11:49:59 MDT 2023
+- todo: go test
+  subtasks:
+  - TestAnalyzer_GroupByCategory
+  - TestAnalyzer_BigSpendersReport
+  - TestAnalyzer_TransactionsFromURLs
+  - TestAnalyzer_TransactionsFromURLsConcurrent
+  - amount.go:Rounded probably does NOT handle float precision well... it is float64
+    tho...
+  - my `go mod tidy` actually cleared `go.mod` file, probably weird localhost backwards
+    compatilble stuff
+  - transaction.go:Transaction:String not clear if FormatUSD or amount currency should
+    not be changed, or even what currency Amount is
+  - transaction.go:Transaction:Sum again doesnt care about Amount currency or vendor/vendee
+    drift
+  - analyzer.go:Analyzer:LargestTransaction doesn't specify how to break ties; stable
+    or latest?
+  - analyzer.go:Analyzer:Add should dedupe transactions added, but transactions.go:FromFile
+    will load duplicate transactions from json file so hmmmm
+  - todo: analyzer.go:Analzyer:Add dedupes each transaction, which is O(n**2)
+    details: |
+      * BUT there's no indicator whether order of the array matters, so it's unsafe for me to sort/heapify that stuff
+      * OR I can store a second copy of all entries in a map, but that risks drift syncing the two
+      * SO I could create a UniqueTransactions struct {
+          transactions []Transaction
+          dedupes map[Transaction]struct{}
+        }
+        but that's just doubling RAM usage in a thing that sounds like it could scale infinitely over time
+        SO I could do a [hash(Transaction)][]*Transaction and compare just a subset. Because it's in RAM and computed live, the hash cardinality could be changed on any release
+        <------------------ if I have time, do this
+  - analyzer.go:Analyzer:Add dedupes but what is a duplicate transaction? Transactions
+    can be pending and then later disappear to have their date updated OR be like
+    pre-charges on credit cards that later disappear
+  - analyzer.go:Analyzer:Add is not concurrency-safe
+  ts: Sun Oct 15 11:52:22 MDT 2023
+- todo: go test
+  subtasks:
+  - TestAnalyzer_GroupByCategory
+  - TestAnalyzer_BigSpendersReport
+  - TestAnalyzer_TransactionsFromURLs
+  - TestAnalyzer_TransactionsFromURLsConcurrent
+  - amount.go:Rounded probably does NOT handle float precision well... it is float64
+    tho...
+  - my `go mod tidy` actually cleared `go.mod` file, probably weird localhost backwards
+    compatilble stuff
+  - transaction.go:Transaction:String not clear if FormatUSD or amount currency should
+    not be changed, or even what currency Amount is
+  - transaction.go:Transaction:Sum again doesnt care about Amount currency or vendor/vendee
+    drift
+  - analyzer.go:Analyzer:LargestTransaction doesn't specify how to break ties; stable
+    or latest?
+  - analyzer.go:Analyzer:Add should dedupe transactions added, but transactions.go:FromFile
+    will load duplicate transactions from json file so hmmmm
+  - todo: analyzer.go:Analzyer:Add dedupes each transaction, which is O(n**2)
+    details: |
+      * BUT there's no indicator whether order of the array matters, so it's unsafe for me to sort/heapify that stuff
+      * OR I can store a second copy of all entries in a map, but that risks drift syncing the two
+      * SO I could create a UniqueTransactions struct {
+          transactions []Transaction
+          dedupes map[Transaction]struct{}
+        }
+        but that's just doubling RAM usage in a thing that sounds like it could scale infinitely over time
+        SO I could do a [hash(Transaction)][]*Transaction and compare just a subset. Because it's in RAM and computed live, the hash cardinality could be changed on any release
+        <------------------ if I have time, do this
+  - analyzer.go:Analyzer:Add dedupes but what is a duplicate transaction? Transactions
+    can be pending and then later disappear to have their date updated OR be like
+    pre-charges on credit cards that later disappear
+  - analyzer.go:Analyzer:Add is not concurrency-safe
+  - analyzer.go:Analyzer:ByCategory probably allocates big slices out the gate and
+    expends to way-too-big-slices by end time. Could do 2 passes (still O(n)) to pre-compute
+    each category's size. Makes code look weird, though. Hm.
+  ts: Sun Oct 15 11:52:25 MDT 2023