Teki DATEV Viewer

A lightweight, read-only desktop application for inspecting and validating DATEV EXTF files before importing them into accounting software.

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Overview

Teki DATEV Viewer is built for entrepreneurs and accounting professionals who need to verify the structure and contents of a DATEV export before handing it off to their accountant or importing it into DATEV-compatible software.

It never modifies, repairs, or writes back to your files. What you see is exactly what is in the file.

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Features

• Drag & drop — open a file by dropping it anywhere on the window
• File summary — encoding, line endings, file size, and record count at a glance
• Header metadata — all parsed DATEV EXTF header fields displayed as key/value pairs
• Booking table — virtualised, scrollable table with:
  • sortable and resizable columns
  • instant full-text search and filtering
  • row and cell selection with clipboard copy (Ctrl+C / ⌘C)
• Validation — structural validation of header and every booking record, with severity indicators (Valid / Warning / Invalid)
• Totals — booking count, total amount, debit total, credit total, warning count, error count
• Error screen — detailed diagnostics for files that cannot be parsed, including the offending line
• Completely offline — no network access, no telemetry, no account required

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Supported Input

Property	Value
Format	DATEV EXTF CSV
Extensions	.csv, .txt
Encodings	UTF-8, UTF-8 with BOM, Windows-1252
Line endings	CRLF, LF

Format compatibility is determined from file contents, not from the file extension.

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Platforms

Platform	Status
macOS (Apple Silicon)	✅ Supported
Windows x64	✅ Supported
Linux	Not supported in v1

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Technology Stack

Layer	Technology
Desktop shell	Tauri 2
UI	React 18 + TypeScript
Business logic	Rust
Bundler	Vite

All parsing, validation, encoding detection, and totals calculation are implemented in Rust (datev_core). The React frontend is responsible for presentation only.

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Architecture

┌─────────────────────────────────────┐
│          React UI (TypeScript)       │
│  Presentation · State · Tables       │
└──────────────┬──────────────────────┘
               │  Tauri IPC
┌──────────────▼──────────────────────┐
│        Rust Backend (Tauri)          │
│  File I/O · Encoding detection       │
└──────────────┬──────────────────────┘
               │
┌──────────────▼──────────────────────┐
│           datev_core (crate)         │
│  Parser · Validator · Calculator     │
│  Domain model · Totals               │
└─────────────────────────────────────┘

datev_core is a standalone Rust library crate. It has no dependency on Tauri or any desktop framework and can be reused or tested independently.

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Development

Prerequisites

Development takes place inside an OrbStack (or Docker) container. The container includes all Rust and Node toolchains.

Build the container image once (or after Dockerfile changes):

docker compose build

Install Node dependencies:

docker compose run --rm dev npm install

Common Commands

Task	Command
TypeScript type-check	docker compose run --rm dev npx tsc --noEmit
Rust compilation check	docker compose run --rm dev cargo check --workspace
Run Rust tests	docker compose run --rm dev cargo test --workspace
Rust linter	docker compose run --rm dev cargo clippy --workspace -- -D warnings
Rust formatter check	docker compose run --rm dev cargo fmt --all -- --check

Note: All of the above commands run inside the container. Do not run them directly on the macOS host.

Running the Desktop App (macOS host only)

npm run tauri dev

This command runs on the macOS host and requires explicit approval before execution.

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Project Structure

teki-datev-viewer/
├── datev_core/          # Standalone Rust library: parser, validator, calculator
│   └── src/
│       ├── lib.rs       # Public API: process_decoded_file()
│       ├── models.rs    # Domain model (DatevFile, BookingRecord, …)
│       ├── parser.rs    # DATEV EXTF CSV parser
│       ├── reader.rs    # File reading, encoding & line-ending detection
│       ├── validator.rs # Structural validation engine
│       └── calculator.rs# Totals calculation
│
├── src-tauri/           # Tauri application shell (Rust)
│
├── src/                 # React frontend (TypeScript)
│   ├── App.tsx          # Application root, state management
│   ├── types.ts         # TypeScript view models
│   └── components/
│       ├── BookingTable.tsx      # Virtualised booking table
│       └── ValidationResults.tsx # Validation status panel
│
├── Dockerfile
├── docker-compose.yml
└── SPECS.md             # Product Requirements Specification

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Validation Rules

File level

• Non-empty, readable file
• Supported and consistent encoding
• Consistent line endings
• Valid CSV structure
• Valid DATEV EXTF identifier in the first field
• Consistent column count across rows

Header

• Mandatory fields present
• Supported DATEV version (510, 700)
• Consultant and client number format
• Valid date values and accounting period range

Booking records

• Mandatory fields present (amount, date, account)
• Amount format (numeric, comma as decimal separator)
• Date format (DDMM / DMM / YYYYMMDD)
• Account and contra-account numeric format and length
• Tax key format and length

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Safety Guarantees

• The application never writes to or modifies an opened file
• The application never auto-corrects or silently repairs malformed input
• The application never makes network requests
• Given identical input, output is always identical (fully deterministic)

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License

Licensed under the Apache License, Version 2.0.