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SEN LLC

I build small, focused tools and ship them open-source. Based in Japan 🇯🇵 with SEN LLC, working through a 100+ portfolio at sen.ltd/portfolio. Posts about TypeScript, AWS, and bugs that taught me so

Location Tokyo, Japan Joined Joined on  Personal website https://sen.ltd

Work

Engineer at SEN LLC

A Math-Expression Parser in 250 Lines of JavaScript — Recursive Descent, Right-Associative ^, and -2^2 = -4
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A Math-Expression Parser in 250 Lines of JavaScript — Recursive Descent, Right-Associative ^, and -2^2 = -4

#javascript #parsers #compilers #webdev
Comments
6 min read

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Computing Moon Phase in 150 Lines of JavaScript — Julian Date, Synodic Month, and SVG Terminator
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Computing Moon Phase in 150 Lines of JavaScript — Julian Date, Synodic Month, and SVG Terminator

#javascript #svg #astronomy #webdev
Comments
7 min read
A Browser IPv4 CIDR Calculator — Three JavaScript Gotchas (uint32 Coercion, /0 Shift Trap, RFC 3021 /31)
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A Browser IPv4 CIDR Calculator — Three JavaScript Gotchas (uint32 Coercion, /0 Shift Trap, RFC 3021 /31)

#javascript #networking #frontend #webdev
1
Comments
5 min read
A Browser-Only Diary in 350 Lines — Month-Grid Math, Code-Point Character Counts, and Streak Boundaries
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A Browser-Only Diary in 350 Lines — Month-Grid Math, Code-Point Character Counts, and Streak Boundaries

#javascript #frontend #webdev #localstorage
Comments
6 min read
Plotting 5 Years of FX in the Browser — Frankfurter API + Hand-Rolled SVG Line Chart
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Plotting 5 Years of FX in the Browser — Frankfurter API + Hand-Rolled SVG Line Chart

#javascript #dataviz #frontend #webdev
Comments
6 min read
Reimplementing path-to-regexp in 100 Lines — Why /users/:id? Almost Never Works the Way You Expect
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Reimplementing path-to-regexp in 100 Lines — Why /users/:id? Almost Never Works the Way You Expect

#javascript #express #regex #webdev
Comments
5 min read
Visualizing A* One Node at a Time — The Tie-Break and Corner-Cut Decisions That Decide How the Demo Looks
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Visualizing A* One Node at a Time — The Tie-Break and Corner-Cut Decisions That Decide How the Demo Looks

#javascript #algorithms #frontend #visualization
1
Comments
6 min read
A 350-Line GLSL Shader Playground in the Browser — WebGL Init, Line-Number-Aware Errors, and URL-Hash Sharing
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A 350-Line GLSL Shader Playground in the Browser — WebGL Init, Line-Number-Aware Errors, and URL-Hash Sharing

#javascript #webgl #frontend #webdev
1
Comments
6 min read
A Browser Ear-Training Trainer in 350 Lines — Equal-Temperament Frequencies and Three Web Audio Footguns
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A Browser Ear-Training Trainer in 350 Lines — Equal-Temperament Frequencies and Three Web Audio Footguns

#javascript #webaudio #music #frontend
Comments
6 min read
A Real-Time Earthquake Map for Japan in 300 Lines of JavaScript — USGS GeoJSON + Leaflet
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A Real-Time Earthquake Map for Japan in 300 Lines of JavaScript — USGS GeoJSON + Leaflet

#javascript #frontend #dataviz #webdev
4
Comments 1
6 min read
Searching Emojis With Casual Japanese Keywords — Why Unicode CLDR's ja Annotations Aren't Enough
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Searching Emojis With Casual Japanese Keywords — Why Unicode CLDR's ja Annotations Aren't Enough

#javascript #japanese #frontend #webdev
Comments
5 min read
Reading Apple Pencil Pressure in the Browser — PointerEvent, getCoalescedEvents(), and the e.pressure Trap
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Reading Apple Pencil Pressure in the Browser — PointerEvent, getCoalescedEvents(), and the e.pressure Trap

#javascript #canvas #frontend #webdev
Comments
7 min read
csv-peek — A 700-Line Rust CLI That Pretty-Prints CSV in the Terminal With a Hand-Rolled RFC 4180 Parser, Type Inference, and One Dependency
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csv-peek — A 700-Line Rust CLI That Pretty-Prints CSV in the Terminal With a Hand-Rolled RFC 4180 Parser, Type Inference, and One Dependency

#rust #cli #csv #terminal
Comments
6 min read
Animating Japan's Population Pyramid From 1950 to 2070 — Why You Need Cohort-Specific Mortality to Get the 1950 Triangle Right
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Animating Japan's Population Pyramid From 1950 to 2070 — Why You Need Cohort-Specific Mortality to Get the 1950 Triangle Right

#javascript #dataviz #frontend #demographics
1
Comments
7 min read
Auto-Furigana in the Browser — Lazy-Loading kuromoji.js's 4 MB Dictionary from a CDN to Annotate Japanese Kanji With Their Readings
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Auto-Furigana in the Browser — Lazy-Loading kuromoji.js's 4 MB Dictionary from a CDN to Annotate Japanese Kanji With Their Readings

#javascript #japanese #nlp #frontend
Comments
6 min read
Building an Interactive Merkle Tree Visualizer — One Byte Changes, the Whole Root Changes, in O(log n) Hops
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Building an Interactive Merkle Tree Visualizer — One Byte Changes, the Whole Root Changes, in O(log n) Hops

#javascript #webdev #crypto #frontend
1
Comments
9 min read
A Live KeyboardEvent Inspector — Settling the `event.key` vs `event.code` Question for JS Shortcuts
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A Live KeyboardEvent Inspector — Settling the `event.key` vs `event.code` Question for JS Shortcuts

#javascript #webdev #frontend #dom
Comments
5 min read
Calling the Anthropic API Directly From the Browser — A 150-Line BYOK Comparison Tool for Opus / Sonnet / Haiku
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Calling the Anthropic API Directly From the Browser — A 150-Line BYOK Comparison Tool for Opus / Sonnet / Haiku

#javascript #anthropic #ai #webdev
1
Comments
5 min read
A 150-Line Rust CLI That Renders QR Codes in Your Terminal — Half-Block Characters Pack Two Modules per Cell
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A 150-Line Rust CLI That Renders QR Codes in Your Terminal — Half-Block Characters Pack Two Modules per Cell

#rust #cli #productivity #devops
Comments
4 min read
The Three Conditions for Seeing the ISS — A Browser-Only Visibility Calculator with Spherical Trigonometry
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The Three Conditions for Seeing the ISS — A Browser-Only Visibility Calculator with Spherical Trigonometry

#javascript #space #math #webdev
Comments 2
5 min read
Sample-Free Drum Synthesis in Web Audio — Building Kick, Snare, and Hi-Hat from Oscillators in 60 Lines
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Sample-Free Drum Synthesis in Web Audio — Building Kick, Snare, and Hi-Hat from Oscillators in 60 Lines

#javascript #webaudio #audio #music
1
Comments
6 min read
A Per-URL TODO List Chrome Extension in 150 Lines — Single-Key chrome.storage Design, URL Normalization, and Three Sync Paths for the Toolbar Badge
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A Per-URL TODO List Chrome Extension in 150 Lines — Single-Key chrome.storage Design, URL Normalization, and Three Sync Paths for the Toolbar Badge

#chrome #javascript #productivity #webdev
1
Comments 1
5 min read
A 150-Line `whentime tokyo london ny` CLI in Rust — and Why You Need IANA tzdata, Not 'UTC + N'
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A 150-Line `whentime tokyo london ny` CLI in Rust — and Why You Need IANA tzdata, Not 'UTC + N'

#rust #cli #productivity #devops
Comments
5 min read
Three Address Checksums, Three Engineering Philosophies — Verifying Base58Check, Bech32m, and EIP-55 in 250 Lines of Browser JS
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Three Address Checksums, Three Engineering Philosophies — Verifying Base58Check, Bech32m, and EIP-55 in 250 Lines of Browser JS

#javascript #crypto #bitcoin #ethereum
Comments
6 min read
A 230-Line Chrome MV3 Extension That Copies the Page Selection as Markdown — Without `<all_urls>`
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A 230-Line Chrome MV3 Extension That Copies the Page Selection as Markdown — Without `<all_urls>`

#chrome #javascript #webdev #productivity
Comments
6 min read
BIP39 in 180 Lines of Vanilla JS — Mnemonic Generation, Validation, Seed Derivation, and the Japanese Wordlist Trap
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BIP39 in 180 Lines of Vanilla JS — Mnemonic Generation, Validation, Seed Derivation, and the Japanese Wordlist Trap

#javascript #crypto #security #webdev
2
Comments 1
6 min read
Why Your Web Audio Tuner Sucks at Low Notes (and What to Use Instead of FFT)
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Why Your Web Audio Tuner Sucks at Low Notes (and What to Use Instead of FFT)

#javascript #webaudio #frontend #audio
Comments
6 min read
GitHub's Stargazers API Caps at 40,000. Here's How Star-History Tools Fake the Rest
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GitHub's Stargazers API Caps at 40,000. Here's How Star-History Tools Fake the Rest

#github #api #javascript #dataviz
Comments
6 min read
A 3.7 KB Othello Engine: Bitboards in Rust, Raw WASM, No wasm-bindgen
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A 3.7 KB Othello Engine: Bitboards in Rust, Raw WASM, No wasm-bindgen

#rust #webassembly #gamedev
Comments
6 min read
I Built a Japanese Poetry Quiz and the Web Speech API Showed Me Its Teeth
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I Built a Japanese Poetry Quiz and the Web Speech API Showed Me Its Teeth

#vue #typescript #webspeechapi #japanese
1
Comments
5 min read
I Finally Understand the SVG d Attribute — Because I Built an Editor for It
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I Finally Understand the SVG d Attribute — Because I Built an Editor for It

#svg #svelte #typescript #webdev
1
Comments
8 min read
I Put the Same Mandelbrot Kernel on the CPU and the GPU — and Watched float32 Crack
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I Put the Same Mandelbrot Kernel on the CPU and the GPU — and Watched float32 Crack

#webgl #javascript #graphics #performance
Comments
6 min read
Why setTimeout Is a Bad Metronome — and What to Use Instead
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Why setTimeout Is a Bad Metronome — and What to Use Instead

#javascript #webaudio #frontend #performance
Comments
5 min read
How Much Faster Is Rust/WASM Than JS for Image Filters? I Ran Both Side by Side and Measured
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How Much Faster Is Rust/WASM Than JS for Image Filters? I Ran Both Side by Side and Measured

#webassembly #rust #javascript #performance
2
Comments
6 min read
A Cross-OS Port Finder in Rust — One CLI, Three Completely Different Data Formats
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A Cross-OS Port Finder in Rust — One CLI, Three Completely Different Data Formats

#rust #cli #linux #networking
Comments
9 min read
A 300-Line GitHub Actions Security Linter: Five Rules That Catch the CVE Patterns
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A 300-Line GitHub Actions Security Linter: Five Rules That Catch the CVE Patterns

#typescript #githubactions #security #cli
Comments
7 min read
A TLS Certificate CLI in Rust — No OpenSSL, No Ceremony, Just a Fail-the-Build Exit Code
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A TLS Certificate CLI in Rust — No OpenSSL, No Ceremony, Just a Fail-the-Build Exit Code

#rust #cli #tls #monitoring
1
Comments
7 min read
CSV Sort, Done Right: Why `sort -t,` Is a Trap and What a 300-Line PHP CLI Does Instead
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CSV Sort, Done Right: Why `sort -t,` Is a Trap and What a 300-Line PHP CLI Does Instead

#php #cli #csv #tutorial
1
Comments
9 min read
Reimplementing fd in 500 lines of Rust (and what I learned about the crates it stands on)
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Reimplementing fd in 500 lines of Rust (and what I learned about the crates it stands on)

#rust #cli #filesystem #tutorial
1
Comments
10 min read
I Built a .env Diff Tool That Refuses to Print Values
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I Built a .env Diff Tool That Refuses to Print Values

#typescript #env #cli #tutorial
1
Comments 1
10 min read
Writing a Regex Playground in 200 Lines of Rust
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Writing a Regex Playground in 200 Lines of Rust

#rust #regex #cli #tutorial
Comments
9 min read
A 10 MB Markdown-to-HTML Service in Rust with pulldown-cmark
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A 10 MB Markdown-to-HTML Service in Rust with pulldown-cmark

#rust #axum #markdown #tutorial
Comments
9 min read
git log --pretty is a tiny DSL: building a zero-dep CHANGELOG generator
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git log --pretty is a tiny DSL: building a zero-dep CHANGELOG generator

#typescript #git #cli #tutorial
Comments
8 min read
Seed Data Without a Dependency Tree: A Tiny PHP CLI Instead of Faker
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Seed Data Without a Dependency Tree: A Tiny PHP CLI Instead of Faker

#php #cli #database #tutorial
Comments
10 min read
sharp on Alpine Without Tears: a 260 MB Image Resize API in Hono
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sharp on Alpine Without Tears: a 260 MB Image Resize API in Hono

#typescript #hono #sharp #tutorial
Comments
9 min read
Writing a tar Inspector in Rust with Three Dependencies
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Writing a tar Inspector in Rust with Three Dependencies

#rust #tar #cli #tutorial
Comments
9 min read
A Tiny PHP Invoice-PDF API: Slim 4, dompdf, and the JPY Rounding Footgun
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A Tiny PHP Invoice-PDF API: Slim 4, dompdf, and the JPY Rounding Footgun

#php #slim #pdf #tutorial
Comments
9 min read
Writing dig in 500 Lines of Rust (with hickory-resolver, the trust-dns successor)
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Writing dig in 500 Lines of Rust (with hickory-resolver, the trust-dns successor)

#rust #dns #cli #tutorial
Comments
10 min read
Writing Cron Semantics in ~100 Lines of TypeScript
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Writing Cron Semantics in ~100 Lines of TypeScript

#typescript #cron #cli #tutorial
2
Comments
8 min read
PHPStan's JSON Output Is Noise. I Wrote a Formatter with a Baseline Diff.
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PHPStan's JSON Output Is Noise. I Wrote a Formatter with a Baseline Diff.

#php #phpstan #cli #tutorial
Comments
9 min read
I Got Tired of Pasting EXPLAIN Plans Into Web Tools, So I Wrote pg-explain-viz
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I Got Tired of Pasting EXPLAIN Plans Into Web Tools, So I Wrote pg-explain-viz

#rust #postgres #cli #tutorial
Comments
10 min read
Discord Slash Commands Without an SDK: ed25519 in 250 Lines of TypeScript
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Discord Slash Commands Without an SDK: ed25519 in 250 Lines of TypeScript

#typescript #hono #discord #tutorial
Comments
10 min read
Writing an HTTP Load Tester That Doesn't Lie About p99
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Writing an HTTP Load Tester That Doesn't Lie About p99

#rust #benchmark #http #tutorial
Comments
8 min read
A Portfolio-Sized URL Shortener in 300 Lines of PHP
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A Portfolio-Sized URL Shortener in 300 Lines of PHP

#php #slim #urlshortener #tutorial
1
Comments
9 min read
Describing Cron Expressions: Why Templates Don't Work, and What Does
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Describing Cron Expressions: Why Templates Don't Work, and What Does

#typescript #cron #cli #tutorial
Comments
8 min read
Writing a YAML Linter in Rust Because YAML Is Secretly Terrible
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Writing a YAML Linter in Rust Because YAML Is Secretly Terrible

#rust #yaml #cli #tutorial
Comments
11 min read
Parsing Prisma Schema By Hand: A 300-Line CLI That Emits Mermaid ER Diagrams
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Parsing Prisma Schema By Hand: A 300-Line CLI That Emits Mermaid ER Diagrams

#typescript #prisma #mermaid #tutorial
Comments
10 min read
Debugging TCP by Building Your Own Logging Proxy in 400 Lines of Rust
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Debugging TCP by Building Your Own Logging Proxy in 400 Lines of Rust

#rust #tokio #networking #tutorial
1
Comments
8 min read
Building an XPath Query Tool in PHP — Slice XML Like a Pro
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Building an XPath Query Tool in PHP — Slice XML Like a Pro

#php #cli #xml #xpath
Comments
8 min read
Building a Changelog Generator in Rust — From Conventional Commits to Keep a Changelog
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Building a Changelog Generator in Rust — From Conventional Commits to Keep a Changelog

#rust #cli #git #changelog
Comments
8 min read
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