CovidSim connector: Refactor param file lookup and fix Canada population density path

Use the US/Canada population density file for Canada, not the European one.
Factor out helper functions to obtain the path for each parameter file,
based on the input region.
Add population density lookup for US territories + Alaska and Hawaii,
although these are not yet supported by the UI and runner.

Author: adityasharad

packages/mrc-ide-covidsim/src/imperial.ts

@@ -45,63 +45,91 @@ export class ImperialModel implements Model {
     this.threadCount = threadCount
   }
 
+  /** Gets the path to the administrative units parameter file for the given region. */
+  private getAdminPath(region: string): string {
+    if (region === 'US') {
+      return path.join(this.dataDir, 'admin_units', 'United_States_admin.txt')
+    } else if (COUNTRY_PARAMS_BY_ISO_CODE[region]) {
+      const { adminFileName } = COUNTRY_PARAMS_BY_ISO_CODE[region]
+      return path.join(this.dataDir, 'admin_units', adminFileName)
+    } else {
+      throw new Error(`Could not find admin file for region ${region}`)
+    }
+  }
+
+  /**
+   * Gets the path to the population density parameter file for the given region.
+   * Europe is used as the default.
+   */
+  private getPopulationDensityPath(region: string, subregion?: string): string {
+    let populationDensityFileName: string
+    if (
+      ['AS', 'GU', 'PR', 'VI'].includes(region) ||
+      (region === 'US' &&
+        ['US-AK', 'US-HI', 'US-AS', 'US-GU', 'US-PR', 'US-VI'].includes(
+          subregion
+        ))
+    ) {
+      populationDensityFileName = 'wpop_us_terr.txt'
+    } else if (['US', 'CA'].includes(region)) {
+      populationDensityFileName = 'wpop_usacan.txt'
+    } else {
+      populationDensityFileName = 'wpop_eur.txt'
+    }
+    return path.join(this.dataDir, 'populations', populationDensityFileName)
+  }
+
+  /**
+   * Gets the path to the pre-parameters template file for the given region.
+   * The UK is used as the default for known regions.
+   */
+  private getPreParametersTemplatePath(region: string): string {
+    let preParamsFileName: string
+    if (region === 'US') {
+      preParamsFileName = 'preUS_R0=2.0.txt'
+    } else if (COUNTRY_PARAMS_BY_ISO_CODE[region]) {
+      preParamsFileName =
+        COUNTRY_PARAMS_BY_ISO_CODE[region].preParamsFileName ??
+        'preUK_R0=2.0.txt'
+    } else {
+      throw new Error(
+        `Could not find pre-parameters template file for region ${region}`
+      )
+    }
+    return path.join(this.dataDir, 'param_files', preParamsFileName)
+  }
+
+  private getSubregionName(region: string, subregion: string): string {
+    if (region === 'US') {
+      return US_SUBREGIONS[subregion]
+    } else if (COUNTRY_PARAMS_BY_ISO_CODE[region]) {
+      const { subregions } = COUNTRY_PARAMS_BY_ISO_CODE[region]
+      return subregions[subregion]
+    } else {
+      throw new Error(`Could not find subregions for region ${region}`)
+    }
+  }
+
   inputs(input: input.ModelInput): ImperialRunnerModelInput {
     const inputFiles = []
-    // Select the correct executable and static inputs based on the region.
-    let adminPath,
-      populationDensityPath,
-      preParametersTemplatePath,
-      subregionName
 
     const modelPath = path.join(this.binDir, 'CovidSim')
 
+    // Select the correct static inputs based on the region.
+    let adminPath = this.getAdminPath(input.region)
+    const populationDensityPath = this.getPopulationDensityPath(
+      input.region,
+      input.subregion
+    )
     const parametersTemplatePath = path.join(
       this.dataDir,
       'param_files',
       'p_NoInt.txt'
     )
-
-    // The US has its own executable, population density file, and pre-params file.
-    if (input.region === 'US') {
-      adminPath = path.join(
-        this.dataDir,
-        'admin_units',
-        'United_States_admin.txt'
-      )
-      populationDensityPath = path.join(
-        this.dataDir,
-        'populations',
-        'wpop_usacan.txt'
-      )
-      preParametersTemplatePath = path.join(
-        this.dataDir,
-        'param_files',
-        'preUS_R0=2.0.txt'
-      )
-      subregionName = US_SUBREGIONS[input.subregion]
-    } else if (COUNTRY_PARAMS_BY_ISO_CODE[input.region]) {
-      const { adminFileName, subregions } = COUNTRY_PARAMS_BY_ISO_CODE[
-        input.region
-      ]
-
-      adminPath = path.join(this.dataDir, 'admin_units', adminFileName)
-      populationDensityPath = path.join(
-        this.dataDir,
-        'populations',
-        'wpop_eur.txt'
-      )
-
-      const preParamsFileName =
-        COUNTRY_PARAMS_BY_ISO_CODE[input.region].preParamsFileName ??
-        'preUK_R0=2.0.txt'
-
-      preParametersTemplatePath = path.join(
-        this.dataDir,
-        'param_files',
-        preParamsFileName
-      )
-      subregionName = subregions[input.subregion]
-    }
+    const preParametersTemplatePath = this.getPreParametersTemplatePath(
+      input.region
+    )
+    const subregionName = this.getSubregionName(input.region, input.subregion)
 
     // Generate the intervention-related pre-parameters based on the input.
     inputFiles.push(preParametersTemplatePath)