go-02-word-frequency

# Word frequency

Implement **`solution.go`** in `package challenge` exporting:

```go
func WordFrequency(text string) map[string]int
```

Count how many times each word occurs in `text` and return the counts in a map.

Rules:

- Split `text` into tokens on **whitespace** (spaces, tabs, newlines).
- For each token, strip any **surrounding ASCII punctuation** (leading and trailing).
  Punctuation in the middle of a token is kept.
- **Lowercase** each word before counting.
- If, after stripping, a token is empty, skip it (do not count an empty string).
- For empty or whitespace-only input, return an **empty, non-nil** map (length 0).

ASCII punctuation is the set of characters for which Go's `unicode.IsPunct` returns true
together with symbols such as `+`, `<`, `=`, etc. For this challenge, treat a byte as
"punctuation to strip" when it is an ASCII byte that is **not** a letter or digit.

Examples:

- `WordFrequency("the cat sat on the mat")` →
  `{"the": 2, "cat": 1, "sat": 1, "on": 1, "mat": 1}`
- `WordFrequency("Hello, hello! HELLO.")` → `{"hello": 3}`
- `WordFrequency("don't stop")` → `{"don't": 1, "stop": 1}` (interior apostrophe kept)
- `WordFrequency("   ")` → `{}` (empty, non-nil map)

package challenge

import (
	"reflect"
	"testing"
)

func TestWordFrequency(t *testing.T) {
	cases := []struct {
		name string
		in   string
		want map[string]int
	}{
		{
			name: "simple repeats",
			in:   "the cat sat on the mat",
			want: map[string]int{"the": 2, "cat": 1, "sat": 1, "on": 1, "mat": 1},
		},
		{
			name: "punctuation and case",
			in:   "Hello, hello! HELLO.",
			want: map[string]int{"hello": 3},
		},
		{
			name: "interior apostrophe kept",
			in:   "don't stop don't",
			want: map[string]int{"don't": 2, "stop": 1},
		},
		{
			name: "tabs and newlines as whitespace",
			in:   "a\tb\nc a",
			want: map[string]int{"a": 2, "b": 1, "c": 1},
		},
		{
			name: "leading and trailing punctuation stripped",
			in:   "(go) [go]; {GO}",
			want: map[string]int{"go": 3},
		},
		{
			name: "digits are words",
			in:   "42 42 forty-two",
			want: map[string]int{"42": 2, "forty-two": 1},
		},
		{
			name: "token that is only punctuation is skipped",
			in:   "hi --- !!! bye",
			want: map[string]int{"hi": 1, "bye": 1},
		},
		{
			name: "empty input",
			in:   "",
			want: map[string]int{},
		},
		{
			name: "whitespace only",
			in:   "   \t \n  ",
			want: map[string]int{},
		},
	}

	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := WordFrequency(c.in)
			if got == nil {
				t.Fatalf("WordFrequency(%q) returned nil map, want non-nil", c.in)
			}
			if !reflect.DeepEqual(got, c.want) {
				t.Errorf("WordFrequency(%q) = %v, want %v", c.in, got, c.want)
			}
		})
	}
}

func TestWordFrequencyEmptyIsNonNil(t *testing.T) {
	got := WordFrequency("")
	if got == nil {
		t.Fatal("WordFrequency(\"\") = nil, want non-nil empty map")
	}
	if len(got) != 0 {
		t.Errorf("WordFrequency(\"\") = %v, want empty map", got)
	}
}

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

The user wants a Go function `WordFrequency` in `package challenge` that counts word frequencies.

Rules:
1. Split on whitespace.
2. Strip surrounding ASCII punctuation.
   - ASCII punctuation is defined as: ASCII byte that is NOT a letter or digit.
   - This means bytes in range [0, 127] that are not [a-z], [A-Z], [0-9].
   - Note: The prompt says "ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc. For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
   - So, if `b` is a byte:
     - If `b` is ASCII (0-127):
       - If `b` is letter or digit: keep.
       - Else: strip.
     - If `b` is not ASCII (>127):
       - The rule specifically says "treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
       - This implies non-ASCII bytes are NOT considered punctuation to strip based on this specific definition? Or does it mean we only care about ASCII bytes for stripping?
       - Let's re-read carefully: "For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
       - This suggests that if a byte is non-ASCII, it is NOT punctuation to strip. It might be part of the word.
       - However, usually "word" implies letters. But I must follow the prompt strictly.
       - Let's check the examples.
       - "don't" -> interior apostrophe kept. Apostrophe is ASCII, not letter/digit. So it's punctuation. But it's in the middle, so it's kept.
       - "Hello, hello!" -> comma and exclamation are ASCII, not letter/digit. Stripped from ends.
       - So the logic for stripping is:
         - While the first byte is ASCII and not letter/digit, remove it.
         - While the last byte is ASCII and not letter/digit, remove it.
3. Lowercase each word.
4. Skip empty tokens after stripping.
5. Return empty non-nil map for empty input.

Implementation details:
- Use `strings.Fields` to split on whitespace.
- Iterate over tokens.
- For each token, strip leading/trailing bytes that satisfy `isPunct(b)`.
  - `isPunct(b)`: `b < 128 && !isLetterOrDigit(b)`
  - `isLetterOrDigit(b)`: `(b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9')`
- Convert to lowercase.
- Update map.

Wait, `strings.ToLower` works on runes. If I have non-ASCII bytes, `strings.ToLower` handles them correctly.
The stripping logic needs to be careful with bytes vs runes.
The prompt says "treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
This implies we look at bytes.
If a token is "café", bytes are c, a, f, é (if UTF-8, é is 0xC3 0xA9).
0xC3 is > 127, so not ASCII. So not punctuation to strip.
0xA9 is > 127, so not ASCII. So not punctuation to strip.
So "café" remains "café". Lowercased to "café".
If token is "hello!", '!' is ASCII, not letter/digit. Strip.
If token is "!hello", '!' is ASCII, not letter/digit. Strip.
If token is "don't", 'd' is letter, 'o' letter, 'n' letter, ''' is ASCII, not letter/digit (punct), 't' letter.
Leading: 'd' is letter, stop.
Trailing: 't' is letter, stop.
Result "don't".

What about "123"? Digits are kept.
What about "123abc"? Kept.
What about "abc123"? Kept.

Let's refine the stripping function.
Input: string `s`.
Output: string `s` with leading/trailing ASCII non-alphanumeric bytes removed.

Algorithm:
1. Convert `s` to byte slice or work with indices.
2. Find start index `i`: while `i < len(s)` and `isStripByte(s[i])`, `i++`.
3. Find end index `j`: while `j > i` and `isStripByte(s[j-1])`, `j--`.
4. Return `s[i:j]`.
5. `isStripByte(b)`: `b < 128 && !((b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9'))`.

Wait, `unicode.IsPunct` is mentioned in the prompt but then overridden by the specific rule "treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
The prompt says: "ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc. For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
This second sentence clarifies the definition for the challenge. It effectively defines the set of bytes to strip.
So I will use the byte-based check.

Let's double check if `strings.Fields` is sufficient.
"Split text into tokens on whitespace (spaces, tabs, newlines)."
`strings.Fields` splits on any Unicode whitespace. The prompt specifies "spaces, tabs, newlines".
Usually `strings.Fields` is safe enough, but strictly speaking, it might split on other Unicode whitespace characters (like non-breaking space).
However, in competitive programming contexts, `strings.Fields` is standard.
If I want to be pedantic about "spaces, tabs, newlines", I could use `strings.Split` with a regex or manual loop, but `strings.Fields` is likely intended.
Actually, `strings.Fields` splits on any sequence of Unicode whitespace.
The prompt says "whitespace (spaces, tabs, newlines)". This is a description of what whitespace means in this context, likely implying standard ASCII whitespace.
Given the strict byte definition for punctuation, maybe I should stick to ASCII whitespace?
But `strings.Fields` is robust. Let's assume standard whitespace behavior is acceptable or that the input only contains ASCII whitespace.
Actually, if I want to be safe, I can use `strings.FieldsFunc` with a predicate that checks for space, tab, newline.
But `strings.Fields` is idiomatic. I'll stick with `strings.Fields` unless there's a reason not to.
Wait, if the input contains non-ASCII whitespace, `strings.Fields` splits on it. The prompt says "Split text into tokens on whitespace (spaces, tabs, newlines)."
This might imply ONLY spaces, tabs, newlines.
Let's look at the punctuation rule again. It's very specific about bytes.
Maybe I should implement a custom splitter or use `strings.Fields` and assume it's fine.
Given the examples are simple, `strings.Fields` is the standard Go way.

Let's write the code.

```go
package
…[48066 chars elided]…
g]int)
	
	// Split text into tokens on whitespace.
	// strings.Fields splits on any Unicode whitespace, which covers spaces, tabs, and newlines.
	tokens := strings.Fields(text)
	
	for _, token := range tokens {
		// Strip surrounding ASCII punctuation
		word := stripPunctuation(token)
		
		// Skip empty tokens
		if word == "" {
			continue
		}
		
		// Lowercase the word
		word = strings.ToLower(word)
		
		// Count frequency
		freq[word]++
	}
	
	return freq
}

// stripPunctuation removes leading and trailing bytes that are ASCII punctuation.
// ASCII punctuation is defined as an ASCII byte that is not a letter or digit.
func stripPunctuation(s string) string {
	b := []byte(s)
	
	// Find the start index after leading punctuation
	start := 0
	for start < len(b) && isPunctByte(b[start]) {
		start++
	}
	
	// Find the end index before trailing punctuation
	end := len(b)
	for end > start && isPunctByte(b[end-1]) {
		end--
	}
	
	return string(b[start:end])
}

// isPunctByte returns true if the byte is an ASCII punctuation character to strip.
// This is defined as an ASCII byte (0-127) that is not a letter or digit.
func isPunctByte(b byte) bool {
	if b >= 128 {
		return false
	}
	// Check if letter (a-z, A-Z) or digit (0-9)
	if (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') {
		return false
	}
	return true
}
```
This is the final answer.

Wait, I should check if the prompt requires `solution.go` to be the only file.
"Implement solution.go in package challenge".
Yes.

I will output the code block.

One last check: "ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc."
This sentence is a bit confusing.
`unicode.IsPunct` returns true for punctuation characters.
`+`, `<`, `=` are symbols (`unicode.IsSymbol`).
So the set is `IsPunct` OR `IsSymbol` (for ASCII).
But then it says "For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit."
This second sentence overrides

[
  {
    "total": 1,
    "answer": "",
    "passed": 0,
    "reasoning": "The user wants a Go function `WordFrequency` in `package challenge` that counts word frequencies.\n\nRules:\n1. Split on whitespace.\n2. Strip surrounding ASCII punctuation.\n   - ASCII punctuation is defined as: ASCII byte that is NOT a letter or digit.\n   - This means bytes in range [0, 127] that are not [a-z], [A-Z], [0-9].\n   - Note: The prompt says \"ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc. For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\n   - So, if `b` is a byte:\n     - If `b` is ASCII (0-127):\n       - If `b` is letter or digit: keep.\n       - Else: strip.\n     - If `b` is not ASCII (>127):\n       - The rule specifically says \"treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\n       - This implies non-ASCII bytes are NOT considered punctuation to strip based on this specific definition? Or does it mean we only care about ASCII bytes for stripping?\n       - Let's re-read carefully: \"For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\n       - This suggests that if a byte is non-ASCII, it is NOT punctuation to strip. It might be part of the word.\n       - However, usually \"word\" implies letters. But I must follow the prompt strictly.\n       - Let's check the examples.\n       - \"don't\" -> interior apostrophe kept. Apostrophe is ASCII, not letter/digit. So it's punctuation. But it's in the middle, so it's kept.\n       - \"Hello, hello!\" -> comma and exclamation are ASCII, not letter/digit. Stripped from ends.\n       - So the logic for stripping is:\n         - While the first byte is ASCII and not letter/digit, remove it.\n         - While the last byte is ASCII and not letter/digit, remove it.\n3. Lowercase each word.\n4. Skip empty tokens after stripping.\n5. Return empty non-nil map for empty input.\n\nImplementation details:\n- Use `strings.Fields` to split on whitespace.\n- Iterate over tokens.\n- For each token, strip leading/trailing bytes that satisfy `isPunct(b)`.\n  - `isPunct(b)`: `b < 128 && !isLetterOrDigit(b)`\n  - `isLetterOrDigit(b)`: `(b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9')`\n- Convert to lowercase.\n- Update map.\n\nWait, `strings.ToLower` works on runes. If I have non-ASCII bytes, `strings.ToLower` handles them correctly.\nThe stripping logic needs to be careful with bytes vs runes.\nThe prompt says \"treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\nThis implies we look at bytes.\nIf a token is \"café\", bytes are c, a, f, é (if UTF-8, é is 0xC3 0xA9).\n0xC3 is > 127, so not ASCII. So not punctuation to strip.\n0xA9 is > 127, so not ASCII. So not punctuation to strip.\nSo \"café\" remains \"café\". Lowercased to \"café\".\nIf token is \"hello!\", '!' is ASCII, not letter/digit. Strip.\nIf token is \"!hello\", '!' is ASCII, not letter/digit. Strip.\nIf token is \"don't\", 'd' is letter, 'o' letter, 'n' letter, ''' is ASCII, not letter/digit (punct), 't' letter.\nLeading: 'd' is letter, stop.\nTrailing: 't' is letter, stop.\nResult \"don't\".\n\nWhat about \"123\"? Digits are kept.\nWhat about \"123abc\"? Kept.\nWhat about \"abc123\"? Kept.\n\nLet's refine the stripping function.\nInput: string `s`.\nOutput: string `s` with leading/trailing ASCII non-alphanumeric bytes removed.\n\nAlgorithm:\n1. Convert `s` to byte slice or work with indices.\n2. Find start index `i`: while `i < len(s)` and `isStripByte(s[i])`, `i++`.\n3. Find end index `j`: while `j > i` and `isStripByte(s[j-1])`, `j--`.\n4. Return `s[i:j]`.\n5. `isStripByte(b)`: `b < 128 && !((b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9'))`.\n\nWait, `unicode.IsPunct` is mentioned in the prompt but then overridden by the specific rule \"treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\nThe prompt says: \"ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc. For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\nThis second sentence clarifies the definition for the challenge. It effectively defines the set of bytes to strip.\nSo I will use the byte-based check.\n\nLet's double check if `strings.Fields` is sufficient.\n\"Split text into tokens on whitespace (spaces, tabs, newlines).\"\n`strings.Fields` splits on any Unicode whitespace. The prompt specifies \"spaces, tabs, newlines\".\nUsually `strings.Fields` is safe enough, but strictly speaking, it might split on other Unicode whitespace characters (like non-breaking space).\nHowever, in competitive programming contexts, `strings.Fields` is standard.\nIf I want to be pedantic about \"spaces, tabs, newlines\", I could use `strings.Split` with a regex or manual loop, but `strings.Fields` is likely intended.\nActually, `strings.Fields` splits on any sequence of Unicode whitespace.\nThe prompt says \"whitespace (spaces, tabs, newlines)\". This is a description of what whitespace means in this context, likely implying standard ASCII whitespace.\nGiven the strict byte definition for punctuation, maybe I should stick to ASCII whitespace?\nBut `strings.Fields` is robust. Let's assume standard whitespace behavior is acceptable or that the input only contains ASCII whitespace.\nActually, if I want to be safe, I can use `strings.FieldsFunc` with a predicate that checks for space, tab, newline.\nBut `strings.Fields` is idiomatic. I'll stick with `strings.Fields` unless there's a reason not to.\nWait, if the input contains non-ASCII whitespace, `strings.Fields` splits on it. The prompt says \"Split text into tokens on whitespace (spaces, tabs, newlines).\"\nThis might imply ONLY spaces, tabs, newlines.\nLet's look at the punctuation rule again. It's very specific about bytes.\nMaybe I should implement a custom splitter or use `strings.Fields` and assume it's fine.\nGiven the examples are simple, `strings.Fields` is the standard Go way.\n\nLet's write the code.\n\n```go\npackage\n…[48066 chars elided]…\ng]int)\n\t\n\t// Split text into tokens on whitespace.\n\t// strings.Fields splits on any Unicode whitespace, which covers spaces, tabs, and newlines.\n\ttokens := strings.Fields(text)\n\t\n\tfor _, token := range tokens {\n\t\t// Strip surrounding ASCII punctuation\n\t\tword := stripPunctuation(token)\n\t\t\n\t\t// Skip empty tokens\n\t\tif word == \"\" {\n\t\t\tcontinue\n\t\t}\n\t\t\n\t\t// Lowercase the word\n\t\tword = strings.ToLower(word)\n\t\t\n\t\t// Count frequency\n\t\tfreq[word]++\n\t}\n\t\n\treturn freq\n}\n\n// stripPunctuation removes leading and trailing bytes that are ASCII punctuation.\n// ASCII punctuation is defined as an ASCII byte that is not a letter or digit.\nfunc stripPunctuation(s string) string {\n\tb := []byte(s)\n\t\n\t// Find the start index after leading punctuation\n\tstart := 0\n\tfor start < len(b) && isPunctByte(b[start]) {\n\t\tstart++\n\t}\n\t\n\t// Find the end index before trailing punctuation\n\tend := len(b)\n\tfor end > start && isPunctByte(b[end-1]) {\n\t\tend--\n\t}\n\t\n\treturn string(b[start:end])\n}\n\n// isPunctByte returns true if the byte is an ASCII punctuation character to strip.\n// This is defined as an ASCII byte (0-127) that is not a letter or digit.\nfunc isPunctByte(b byte) bool {\n\tif b >= 128 {\n\t\treturn false\n\t}\n\t// Check if letter (a-z, A-Z) or digit (0-9)\n\tif (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') {\n\t\treturn false\n\t}\n\treturn true\n}\n```\nThis is the final answer.\n\nWait, I should check if the prompt requires `solution.go` to be the only file.\n\"Implement solution.go in package challenge\".\nYes.\n\nI will output the code block.\n\nOne last check: \"ASCII punctuation is the set of characters for which Go's unicode.IsPunct returns true together with symbols such as +, <, =, etc.\"\nThis sentence is a bit confusing.\n`unicode.IsPunct` returns true for punctuation characters.\n`+`, `<`, `=` are symbols (`unicode.IsSymbol`).\nSo the set is `IsPunct` OR `IsSymbol` (for ASCII).\nBut then it says \"For this challenge, treat a byte as 'punctuation to strip' when it is an ASCII byte that is not a letter or digit.\"\nThis second sentence overrides",
    "test_error": "no code extracted from response"
  },
  {
    "total": 2,
    "answer": "```go\npackage challenge\n\nimport (\n\t\"strings\"\n)\n\nfunc WordFrequency(text string) map[string]int {\n\tresult := make(map[string]int)\n\n\ttokens := strings.Fields(text)\n\n\tfor _, token := range tokens {\n\t\t// Strip surrounding ASCII punctuation\n\t\t// ASCII punctuation = any ASCII byte that is NOT a letter or digit\n\n\t\tstart := 0\n\t\tfor start < len(token) {\n\t\t\tb := token[start]\n\t\t\tif (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') {\n\t\t\t\tbreak\n\t\t\t}\n\t\t\tstart++\n\t\t}\n\n\t\tend := len(token)\n\t\tfor end > start {\n\t\t\tb := token[end-1]\n\t\t\tif (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9') {\n\t\t\t\tbreak\n\t\t\t}\n\t\t\tend--\n\t\t}\n\n\t\tif start >= end {\n\t\t\tcontinue\n\t\t}\n\n\t\tword := strings.ToLower(token[start:end])\n\t\tresult[word]++\n\t}\n\n\treturn result\n}\n```",
    "passed": 2,
    "reasoning": "The user wants me to implement a Go function `WordFrequency` in a file `solution.go` in package `challenge`.\n\nLet me analyze the requirements:\n1. Split text on whitespace (spaces, tabs, newlines)\n2. Strip surrounding ASCII punctuation from each token\n3. Lowercase each word\n4. Skip empty tokens after stripping\n5. Return empty non-nil map for empty/whitespace-only input\n6. ASCII punctuation = any ASCII byte that is NOT a letter or digit\n\nLet me write the solution:\n\n```go\npackage challenge\n\nimport (\n\t\"strings\"\n\t\"unicode\"\n)\n\nfunc WordFrequency(text string) map[string]int {\n\tresult := make(map[string]int)\n\t\n\ttokens := strings.Fields(text)\n\t\n\tfor _, token := range tokens {\n\t\t// Strip surrounding ASCII punctuation\n\t\t// ASCII punctuation = ASCII byte that is not a letter or digit\n\t\t\n\t\t// Find first non-punctuation character\n\t\tstart := 0\n\t\tfor start < len(token) {\n\t\t\tb := token[start]\n\t\t\tif b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z' || b >= '0' && b <= '9' {\n\t\t\t\tbreak\n\t\t\t}\n\t\t\tstart++\n\t\t}\n\t\t\n\t\t// Find last non-punctuation character\n\t\tend := len(token)\n\t\tfor end > start {\n\t\t\tb := token[end-1]\n\t\t\tif b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z' || b >= '0' && b <= '9' {\n\t\t\t\tbreak\n\t\t\t}\n\t\t\tend--\n\t\t}\n\t\t\n\t\t// If empty after stripping, skip\n\t\tif start >= end {\n\t\t\tcontinue\n\t\t}\n\t\t\n\t\tword := strings.ToLower(token[start:end])\n\t\tresult[word]++\n\t}\n\t\n\treturn result\n}\n```\n\nWait, let me re-read the requirement about ASCII punctuation:\n\n\"ASCII punctuation is the set of characters for which Go's `unicode.IsPunct` returns true together with symbols such as `+`, `<`, `=`, etc. For this challenge, treat a byte as \"punctuation to strip\" when it is an ASCII byte that is **not** a letter or digit.\"\n\nSo the definition is clear: any ASCII byte that is NOT a letter or digit is punctuation to strip. That means:\n- Letters: a-z, A-Z\n- Digits: 0-9\n- Everything else (including space, but we're already splitting on whitespace) is punctuation\n\nMy implementation looks correct. Let me verify with the examples:\n\n1. `\"the cat sat on the mat\"` → tokens: [\"the\", \"cat\", \"sat\", \"on\", \"the\", \"mat\"] → no punctuation to strip → {\"the\": 2, \"cat\": 1, \"sat\": 1, \"on\": 1, \"mat\": 1} ✓\n\n2. `\"Hello, hello! HELLO.\"` → tokens: [\"Hello,\", \"hello!\", \"HELLO.\"] → strip punctuation → [\"Hello\", \"hello\", \"HELLO\"] → lowercase → [\"hello\", \"hello\", \"hello\"] → {\"hello\": 3} ✓\n\n3. `\"don't stop\"` → tokens: [\"don't\", \"stop\"] → ' is not a letter or digit, but it's in the middle, so we only strip leading/trailing → \"don't\" stays as \"don't\" → {\"don't\": 1, \"stop\": 1} ✓\n\n4. `\"   \"` → tokens: [] → empty map ✓\n\nThis looks correct. Let me output the solution.\n",
    "test_error": ""
  }
]