From 912ca51cd6ace312dfd79875999d0cb1415353d5 Mon Sep 17 00:00:00 2001
From: Guntis Smaukstelis <gsmaukstelis@evolution.com>
Date: Thu, 30 Jan 2025 15:53:26 +0200
Subject: [PATCH] Parse grib file initial attempt

---
 .gitignore                           |  1 +
 README.md                            |  3 ++
 src/main/scala/grib/Grib.scala       |  6 +++
 src/main/scala/grib/GribParser.scala | 59 ++++++++++++++++++++++++++++
 4 files changed, 69 insertions(+)
 create mode 100644 src/main/scala/grib/Grib.scala
 create mode 100644 src/main/scala/grib/GribParser.scala

diff --git a/.gitignore b/.gitignore
index 7d22581..bf27d1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@ src/main/resources/application.conf
 postgres/
 
 target/
+data/
 project/target/
 project/project/target/
 .bsp
diff --git a/README.md b/README.md
index e809fa6..38d932d 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,9 @@ cd web/
 npm run dev
 ```
 
+Run one file
+```sbt "runMain grib.GribParser"```
+
 ## Backend Tech:
 - scala: cats-effects, http4s, fs2, circe, scalatest
 - postgres
diff --git a/src/main/scala/grib/Grib.scala b/src/main/scala/grib/Grib.scala
new file mode 100644
index 0000000..2ab327a
--- /dev/null
+++ b/src/main/scala/grib/Grib.scala
@@ -0,0 +1,6 @@
+package grib
+case class Grib(
+   version: Int,
+   discipline: Int,
+   length: Long,
+ )
\ No newline at end of file
diff --git a/src/main/scala/grib/GribParser.scala b/src/main/scala/grib/GribParser.scala
new file mode 100644
index 0000000..ed43521
--- /dev/null
+++ b/src/main/scala/grib/GribParser.scala
@@ -0,0 +1,59 @@
+package grib
+
+import cats.effect._
+import cats.effect.unsafe.implicits.global
+import fs2.io.file.{Files, Path}
+
+import java.nio.ByteBuffer
+
+// sbt "runMain grib.GribParser"
+object GribParser {
+  def parseFile(path: Path): IO[List[Grib]] = {
+    def loop(ptr: Long, acc: List[Grib]): IO[List[Grib]] = {
+      for {
+        fileSize <- Files[IO].size(path)
+        result <- if (ptr >= fileSize) {
+          IO.pure(acc.reverse)
+        } else {
+          for {
+            grib <- parseGrib(path, ptr)
+            nextResult <- loop(ptr + grib.length, grib :: acc)
+          } yield nextResult
+        }
+      } yield result
+    }
+
+    loop(0L, List.empty)
+  }
+
+  def parseGrib(path: Path, ptr: Long): IO[Grib] = {
+    for {
+      bytes <- readBytes(path, ptr, 16)
+      identifier = new String(bytes.take(4), "ASCII")
+      discipline = bytes(6)
+      version = bytes(7)
+      length = ByteBuffer.wrap(bytes.slice(8, 16)).getLong
+      _ <- IO.println(identifier, version, discipline, length)
+      grib = Grib(version, discipline, length)
+    } yield grib
+  }
+
+
+  def readBytes(path: Path, ptr: Long, len: Int): IO[Array[Byte]] = {
+    Files[IO].readRange(path, 4096, ptr, ptr + len)
+      .compile
+      .to(Array)
+  }
+
+  def main(args: Array[String]): Unit = {
+    val fileName = "data/HARMONIE_DINI_SF_2025-01-24T030000Z_2025-01-26T010000Z.grib"
+    val path = Path(fileName)
+
+    // Example of running the effect
+    val program = for {
+      _ <- parseFile(path)
+    } yield ()
+
+    program.unsafeRunSync()
+  }
+}
\ No newline at end of file