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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

Tokyo, Japan Joined on Apr 10, 2026 https://sen.ltd

Work

Engineer at SEN LLC

Badges

2 Week Community Wellness Streak

1 Week Community Wellness Streak

Writing Debut

Skills/Languages

JavaScript, TypeScript, AWS, Python, Node.js

Currently hacking on

Shipping a 100+ OSS portfolio at sen.ltd/portfolio

281 posts published

6 comments written

19 tags followed

Building an Infinite-Sequence CLI in Haskell — Laziness and Lists That Refer to Themselves, plus a Prime Sieve and Hamming Numbers

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Building an Infinite-Sequence CLI in Haskell — Laziness and Lists That Refer to Themselves, plus a Prime Sieve and Hamming Numbers

#haskell #functional #algorithms #cli

3 min read

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Building a Leftist Heap CLI in OCaml — It's All merge, a Persistent (Immutable) Priority Queue, and an O(log n) Right Spine

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Building a Leftist Heap CLI in OCaml — It's All merge, a Persistent (Immutable) Priority Queue, and an O(log n) Right Spine

#ocaml #functional #algorithms #datastructures

3 min read

Building a Fenwick Tree (BIT) CLI in Zig — It All Hangs on i & -i: O(log n) Prefix Sums, Point Updates, and Binary-Lifted Select

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Building a Fenwick Tree (BIT) CLI in Zig — It All Hangs on i & -i: O(log n) Prefix Sums, Point Updates, and Binary-Lifted Select

#zig #algorithms #datastructures #cli

3 min read

Building an Aho–Corasick CLI in Go — Match a Whole Dictionary in One Pass, Failure Links as Generalized KMP, and Every Overlapping Match

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Building an Aho–Corasick CLI in Go — Match a Whole Dictionary in One Pass, Failure Links as Generalized KMP, and Every Overlapping Match

#go #algorithms #strings #cli

3 min read

Building a Cuckoo Filter CLI in Rust — the Deletion a Bloom Filter Can't Do, Partial-Key Cuckoo Hashing, and a High Load Factor via Relocation

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Building a Cuckoo Filter CLI in Rust — the Deletion a Bloom Filter Can't Do, Partial-Key Cuckoo Hashing, and a High Load Factor via Relocation

#rust #algorithms #datastructures #cli

3 min read

Building a t-digest CLI in Go — Stream p50/p99/p99.9 in a Few KB, a Scale Function That Keeps Tails Sharp, Exact min/max, and Mergeable Digests

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Building a t-digest CLI in Go — Stream p50/p99/p99.9 in a Few KB, a Scale Function That Keeps Tails Sharp, Exact min/max, and Mergeable Digests

#go #algorithms #datastructures #cli

3 min read

Building a Count-Min Sketch CLI in Rust — Sizing from a Target Error, d Counters from Two Hashes, No Underestimates, and Mergeable Sketches

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Building a Count-Min Sketch CLI in Rust — Sizing from a Target Error, d Counters from Two Hashes, No Underestimates, and Mergeable Sketches

#rust #algorithms #datastructures #cli

4 min read

Building a HyperLogLog CLI in Go — One Billion Distinct in 16 KB, a Harmonic Mean, a Hash Finalizer, and Mergeable Sketches

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Building a HyperLogLog CLI in Go — One Billion Distinct in 16 KB, a Harmonic Mean, a Hash Finalizer, and Mergeable Sketches

#go #algorithms #datastructures #cli

4 min read

Building a Bloom Filter CLI in Rust — Sizing from a Target FP Rate, k Hashes from Two, and Guaranteed No False Negatives

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Building a Bloom Filter CLI in Rust — Sizing from a Target FP Rate, k Hashes from Two, and Guaranteed No False Negatives

#rust #algorithms #datastructures #cli

4 min read

Building a CSS Specificity Calculator — Counting (a,b,c) and the :is() / :where() Trap Most Implementations Fall Into

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Building a CSS Specificity Calculator — Counting (a,b,c) and the :is() / :where() Trap Most Implementations Fall Into

#javascript #css #webdev #frontend

5 min read

A Shortest-Path CLI in Rust — Making a Min-Heap from a Max-Heap, Path Reconstruction, and Rejecting Negative Weights

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A Shortest-Path CLI in Rust — Making a Min-Heap from a Max-Heap, Path Reconstruction, and Rejecting Negative Weights

#rust #algorithms #graphs #cli

4 min read

Building a WCAG Contrast Checker — Relative Luminance, sRGB Gamma, and the Step Most Implementations Skip

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Building a WCAG Contrast Checker — Relative Luminance, sRGB Gamma, and the Step Most Implementations Skip

#javascript #a11y #webdev #css

4 min read

Implementing Huffman Coding in Go — Merging the Two Rarest Nodes Builds the Optimal Prefix-Free Code

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Implementing Huffman Coding in Go — Merging the Two Rarest Nodes Builds the Optimal Prefix-Free Code

#go #algorithms #compression #cli

4 min read

Building a Base Converter — BigInt for Unlimited Digits, and Detecting Repeating Fractions

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Building a Base Converter — BigInt for Unlimited Digits, and Detecting Repeating Fractions

#javascript #bigint #algorithms #webdev

4 min read

A Topological-Sort CLI in Python — Kahn's Algorithm and a DFS That Names the Cycle

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A Topological-Sort CLI in Python — Kahn's Algorithm and a DFS That Names the Cycle

#python #algorithms #cli #graphs

4 min read

A Fuzzy-Finder CLI in Go — Two-Row Edit Distance, Unicode Correctness, and Mutex-Free Concurrency by Index

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A Fuzzy-Finder CLI in Go — Two-Row Edit Distance, Unicode Correctness, and Mutex-Free Concurrency by Index

#go #algorithms #cli #concurrency

4 min read

Building an Expression Calculator in Rust — Pratt Parsing Turns Precedence into Two Numbers, and Rationals Kill Float Error

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Building an Expression Calculator in Rust — Pratt Parsing Turns Precedence into Two Numbers, and Rationals Kill Float Error

#rust #parsing #cli #math

5 min read

Building a Unit Converter Taught Me Why Temperature Is Special — Linear Factors vs. Affine Transforms

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Building a Unit Converter Taught Me Why Temperature Is Special — Linear Factors vs. Affine Transforms

#javascript #math #webdev #tools

4 min read

Visualizing git log in the Browser — Punchcard Heatmaps and the Timezone Trap

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Visualizing git log in the Browser — Punchcard Heatmaps and the Timezone Trap

#javascript #git #dataviz #webdev

5 min read

Explaining SQL in Plain English with a Recursive-Descent Parser — and Why Logical Execution Order Is the Real Lesson

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Explaining SQL in Plain English with a Recursive-Descent Parser — and Why Logical Execution Order Is the Real Lesson

#javascript #sql #parsing #webdev

4 min read

A Scatter-Plot Explorer for World Statistics — Log Scales and Hand-Rolled Pearson Correlation

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A Scatter-Plot Explorer for World Statistics — Log Scales and Hand-Rolled Pearson Correlation

#javascript #dataviz #statistics #frontend

5 min read

A Hello World Museum for 84 Languages — and the Round-Trip Test That Makes a Tiny Syntax Highlighter Trustworthy

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A Hello World Museum for 84 Languages — and the Round-Trip Test That Makes a Tiny Syntax Highlighter Trustworthy

#javascript #programming #testing #frontend

4 min read

A Choropleth Map Without Map Paths — Visualizing Japan's Minimum Wages with a Tile Grid

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A Choropleth Map Without Map Paths — Visualizing Japan's Minimum Wages with a Tile Grid

#javascript #dataviz #svg #frontend

5 min read

A Quiz Is Only as Good as Its Wrong Answers — Building Algorithmic Distractor Generation for a Japanese Idiom Quiz

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A Quiz Is Only as Good as Its Wrong Answers — Building Algorithmic Distractor Generation for a Japanese Idiom Quiz

#javascript #algorithms #testing #frontend

5 min read

An In-Browser Image Format Converter in 500 Lines — and Why PNG, JPEG, WebP, AVIF Are More Different Than You Think

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An In-Browser Image Format Converter in 500 Lines — and Why PNG, JPEG, WebP, AVIF Are More Different Than You Think

#javascript #canvas #webdev #frontend

5 min read

A World Timezone Map in 500 Lines — and Why Time Zones Aren't Determined by Longitude

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A World Timezone Map in 500 Lines — and Why Time Zones Aren't Determined by Longitude

#javascript #timezone #webdev #frontend

6 min read

A Japanese Holiday Calendar in 500 Lines — and Why the Spring Equinox Date Isn't Written in Japanese Law

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A Japanese Holiday Calendar in 500 Lines — and Why the Spring Equinox Date Isn't Written in Japanese Law

#javascript #calendar #webdev #frontend

6 min read

A Canvas Favicon Generator in 500 Lines — and Why a Single drawImage() Downscale Always Looks Blurry

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A Canvas Favicon Generator in 500 Lines — and Why a Single drawImage() Downscale Always Looks Blurry

#canvas #javascript #webdev #frontend

6 min read

Building a clamp() Calculator for Fluid Typography — Linear Interpolation + the Accessibility Trick Most Tools Miss

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Building a clamp() Calculator for Fluid Typography — Linear Interpolation + the Accessibility Trick Most Tools Miss

#css #javascript #webdev #a11y

5 min read

Building a Visual cubic-bezier Editor in 500 Lines — Why Browsers Binary-Search the Inverse

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Building a Visual cubic-bezier Editor in 500 Lines — Why Browsers Binary-Search the Inverse

#css #javascript #webdev #frontend

6 min read

Try the Tech Radar #6 (Final) — Semantic Layer in 200 Lines: One Definition, Many SQL Consumers

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Try the Tech Radar #6 (Final) — Semantic Layer in 200 Lines: One Definition, Many SQL Consumers

#data #sql #webdev #javascript

5 min read

Try the Tech Radar #5 — Mutation Testing in 500 Lines of Vanilla JS (and Why High Coverage Lies)

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Try the Tech Radar #5 — Mutation Testing in 500 Lines of Vanilla JS (and Why High Coverage Lies)

#testing #javascript #webdev #frontend

6 min read

Try the Tech Radar #4 — Server-driven UI in a 12-Widget Vanilla JS Interpreter

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Try the Tech Radar #4 — Server-driven UI in a 12-Widget Vanilla JS Interpreter

#ui #javascript #webdev #frontend

6 min read

Try the Tech Radar #3 — JSON Schema LLM Prompt, Visualised

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Try the Tech Radar #3 — JSON Schema LLM Prompt, Visualised

#llm #jsonschema #webdev #javascript

5 min read

Try the Tech Radar #2 — Markdown Typst Converter (Typst's Syntax Is Closer to Markdown Than LaTeX)

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Try the Tech Radar #2 — Markdown Typst Converter (Typst's Syntax Is Closer to Markdown Than LaTeX)

#typst #markdown #javascript #webdev

5 min read

Try the Tech Radar #1 — TOON Cuts JSON Token Cost by 71% for LLM Context

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Try the Tech Radar #1 — TOON Cuts JSON Token Cost by 71% for LLM Context

#json #llm #webdev #javascript

5 min read

A Visual CSS Grid Builder in 500 Lines — How fr, minmax, and repeat Actually Compute Sizes

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A Visual CSS Grid Builder in 500 Lines — How fr, minmax, and repeat Actually Compute Sizes

#css #javascript #webdev #frontend

4 min read

A Click-to-Build Regex Builder in 500 Lines of Vanilla JS — Token Model and Live Highlight Internals

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A Click-to-Build Regex Builder in 500 Lines of Vanilla JS — Token Model and Live Highlight Internals

#javascript #regex #webdev #devtools

5 min read

A Visual CSS @keyframes Editor in 500 Lines — Plus the 'Same Animation Won't Restart' Trap and How to Fix It

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A Visual CSS @keyframes Editor in 500 Lines — Plus the 'Same Animation Won't Restart' Trap and How to Fix It

#css #javascript #animation #webdev

5 min read

Tetris in Vanilla JS — SRS Rotation, 7-Bag Randomizer, and Why You Should Separate Game Logic from Rendering

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Tetris in Vanilla JS — SRS Rotation, 7-Bag Randomizer, and Why You Should Separate Game Logic from Rendering

#javascript #gamedev #canvas #webdev

6 min read

An npm Downloads Comparison Chart in 300 Lines of Vanilla JS — Nice-Tick Math and API-Direct Fetch

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An npm Downloads Comparison Chart in 300 Lines of Vanilla JS — Nice-Tick Math and API-Direct Fetch

#javascript #npm #dataviz #webdev

5 min read

Building a Mini Tailwind-to-CSS Converter — How Utility Class Names Map to Real CSS

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Building a Mini Tailwind-to-CSS Converter — How Utility Class Names Map to Real CSS

#tailwindcss #css #javascript #webdev

5 min read

Building a Chord Progression Generator in the Browser — Music Theory in JS, Sound via Web Audio API

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Building a Chord Progression Generator in the Browser — Music Theory in JS, Sound via Web Audio API

#webaudio #javascript #music #frontend

5 min read

Visualizing 4 Sorting Algorithms with JavaScript Generators — O(n ) vs O(n log n) in Real Time

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Visualizing 4 Sorting Algorithms with JavaScript Generators — O(n ) vs O(n log n) in Real Time

#javascript #algorithms #webdev #beginners

5 min read

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

6 min read

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

9 min read

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